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OEOLOdlOAL  AND  NATUUAL  HISTORY  SURVEY  OF  Ml 
N.  H.  WINCHBLL,  State  OeolO£lst. 


■  MFB.F5 
L4£. 


BULLETIN  No.  8. 


I. 


THE  ANORTHOSYTES  OF  THE 


Minnesota  Coast  of  Lake  Snperior. 


II. 


THE  LACCOLITIC  SILLS  OF  THE 


North- West   Coast   of  Lake   Superior. 


BY  ANDREW  i\  LAWSON, 

ASSOCIATE     PROB'ESSOR     OP     GEOLOGY  AND     MINERALOGY, 
IN  THE   UNIVERSITY   OF  CALIFORNIA. 


WITH  A  PREFATORY  NOTE  ON 

THE  NOBIAN  OF  THE  NORTHAVEST, 

BY   N.    H.    WINCHELL. 


MINNEAPOLIS: 

HAKRISON  &  SMITH,  STATB  PKtNTERS. 
1893. 


:/ 


GEOLfxaOAI.  AND  NATt'KAL  HISTORY  SURVEY  OK  MINNESOTA. 
N.  H.  WINCHELL,  State  Geologist. 


BULLETIN  \(K  il. 


T. 


THE  ANOP.THOSYTES  OF  THE 


Miuiiesotci  Coast  of  Lake  Superior. 


IT. 


THE  LACCOLITIC  SILLS  OF  THE 


North-AYest   Coast   of  Lake   Superior. 


BY  ANDREW  V.  L.VWSOX. 

ASSOCIATE     PROFESSOR     OF     GEOLOGY  AND     MINERALOGY, 
IN   THE   UNIVERSITY   OF   CALIFORNIA, 

WITH  A  PREFATORY  NOTE  ON 

THE  NORTAX  OF  THE  XORTHWEST, 

BY    N.    H.    WINCHELL. 


MINNEAPOLIS: 

HARRISON  &  SMITH,   STATE   PRINTERS. 

I89;t. 


CONTEN1\S. 


THE  NOKIAN  OF  THE  NORTIIM  EST. 

PREFATORY  NOTE. 


Introduction. 
IN'tni^'raphiciil  Chiiraotnrs: 

Irvine's  Description. 

Supplt'racntary  Description. 

Tile  Constituent  Feldspar. 

Optical  Measurements. 

Contlrinatory  Tests. 

Rock  Composed  of  Labradorite. 

Rock  Composed  of  Anorthite. 

Chemical  Analyses. 

The  Name  Anorthosyte. 

Accessory  Constituents  of  the  Anortho.syto. 

Interp(»sition8. 
Distribution  and  Mode  of  Occurrence  of  the  Anorthosyte- 

Two  Modes  of  Occurrenc'e. 

Occurrence  near  Encampment  Island. 

Vicinity  of  Split-rock  Point. 

Irvin(;"s  Views  on  the  Occurrences  at  Split-rock. 

Winchell's  \'iews. 

Anorthosyte  confounded  with  Keweenlan  Eruiitives 

Occurrencesat  IJeaverBay. 

.Shore  below  1  leaver  Hay. 

liaptisru  River 

On  tlie  s|(,i)e  of  Saw-Teeth. 

Carlton  l\!ak. 
GeoloK'ical  relations  of  the  Anorthosyte. 

Pre-Keweenian  a^'e 

I)ome(l  and  humiuocky  character  of  the  pre-Keweenian  surface 
Interval  o!  erosion. 
Al)sence  of  tlie  Animikie. 
Shallowness  of  the  Kewt!i;nian. 
Correlation  and  name  of  the  formation. 

Introduction: 

General  note. 

Earlier  descriptions. 

Dissent  from  former  views. 

Views  here  advanced. 
Petrographical  character  of  the  trap  sheets. 

Constancy  of  character. 

PetroKrapliical  ditl'erentiation. 
Some  broad  features  of  the  trap  sheets: 

Their  simplicity. 

Absence  of  pyroclastic  rocks. 

Absence  of  How  structure. 

The  enclosing  rocks.  • 

Intersection  of  strata  by  the  sheets.  ;• 

Passage  of  sheets  to  tlie  horizon  of  the  Keweenian 
Lower  contact  of  the  Trap  Sheets.  '  * 

Upper  contact. 

Alteration  of  enclosing  rocks. 
Summary. 
Geological  consequences. 


1 


\ 


THE  NORIAN  OP  THE  NORTHWEST. 

PKEPATOUV  NOTE  BY  N.  H    WINCHKf.T.. 

Among  the  problems  which  were  named  in  instructions  given 
to  Dr.  Lawson  when  he  entered  upon  the  season's  work  for  the 
Minnesota  Geological  and  Natural  History  Survey  along  the 
northern  coast  of  lake  Superior,  one  was  the  following — To 
determine  if  possible  the  date  and  stratigrapliic  relations  of  the 
gabbro  invasion.  How  well  he  has  answered  that  question  his 
report  on  the  "Anorthosytes  of  the  Minnesota  coast  of  lake 
Superior"  bears  ample  testimony.  In  order,  however,  that  the 
imjiortant  bearing  which  his  results  have  on  the  geology  of  the 
northeastern  part  of  the  state,  may  be  made  more  ai)parent,  it 
is  deemed  best  to  preface  this  publication  with  a  short  exposi- 
tion of  the  history  of  the  investigation,  and  a  few  i)aragraphs 
on  the  extent  of  the  gabbro  rocks  and  on  the  significance  of  the 
term  gabbro. 

The  reports  of  the  late  survey  of  Wisconsin,  particularly 
those  portions  contributed  by  professors  Pumpelly  and  Irving, 
treat  largely  of  a  terrane  which  they  placed  at  the  bottom  of 
the  then  called  "Keweenawan",  and  which  they  designated 
by  the  general  term  gabbro.  This  name  had  already  been  given 
by  Prof.  Streng  to  the  "Rice  Point  rock"  which  is  seen  abund- 
antly at  Duluth,*  and  by  Dr.  J.  H.  Kloos  in  1871,  who  had  pub- 
lished his  preliminary  field-studies. +  Again  in  1877,  Dr.  Kloos 
gives  further  details  of  the  distribution  and  microscopical  char- 
acter of  this  rock.];  He  calls  attention  to  "two  totally  unlike 
crystalline  rocks"  at  Duluth,  one  of  which  is  the  "gabbro  or 
hyi)ersthene  rock",  which  he  says  had  since  the  examination 
by  Prof.  Streng,  actually  proven  to  be  gabbro,  and  which  has 
an  "enormous  preponderance  of  labrador  plagioclase",  and 
great  paucity  of  other  elements  which  are  "with  difficulty  de- 
tected between  the  feldspar  crystals,  and  can  only  be  dis- 
tinguished with  sufficient  clearness. in  thin  sections".       The 


•  Neues  Jiilirbueli,  IH77.    See  a,  trausliitioti  in  tlie  Ek'vont\i  Minnesota  report  1882. 

t  Zeitschrlft  der  Deiitselien  Ueoiosist'heii  Genellscliaft.  p.  417;  triiiisliitlon  in  tlie 
Teiitli  Minnesota  report.  Compare  pajre  1!>4.  where  tlie  Duliitli  rock  is  said  to  liave 
•'a  reseml)lance  to  gabbro.  or  liyperstlieny  te". 

%  Zeitsolirift  der  Uesellscliatt  fllrErdUurKle  zu  Berlin.  Bd.  XII,  1877.  Translation  in 
tli.e  Nineteenth  annual  report.    Minnesota  survey,  pp.  81—121. 


IV 


HULI.IiTIN    NO.    VI 11. 


other  cry'^tfillinc  rock  dnst^riltcd  by  l\loos  al  Diiliitli  is  tho 
•'porphyry-lilco  mt'lapliyr".  whicli  appears  ii(;ar  tlio  lake  slioro. 
Ho  calls  attention  to  tlie  contrast  wliieli  it  i)rosents  to  th«? 
^'abhro,  and  to  its  similarity  with  I  lie  rnck  at  St.  (Jroix  Falls. 
Tliis  rock  is  not  only  i)orphyritic  l)ut  also  amyj^'^daloidal.  The 
exa<'t  field  relati()n>  l)ei\veen  these  two  he  (!ould  mA  niakc^  out. 
altlHiu^''h  it  has  since  been  asc(!i"taiiied  that  the  nielaphyr  lies 
above  the  <ral)bro. 

The  Wisconsin  ^eolo<fists  have  do.scribod  what  i)lainly  is  the 
same  formation.  It  occurs  in  the  "Had  rivtjr  re«?ion"".  and  its 
areal  (listrii)ution  is  repn'st^iited  on  slieet  XXII.  (accompanyin<^ 
Vol.  111.  oi  that  report ).  (Jf  the  three  chissos  into  which  tlio 
rocksof  the  Keweenawan  system  arodixided  by  i'rof.  I'limpelly. 
the  tliird  is  "(iranular.  ])laj^ioclase  dialla^e  rocks",  and  in 
this  class  he  i)laces  solely  "^alibro"',*  but  reco^'nizin^  some 
variations.  Prof.  l^um])elly  considers  tliat  phase  which  Strong 
named  "liornl)lende  <;abl)ro",  as  one  of  the  variations  of  his 
series.  In  addition  to  the  soda-lime  l'eldsi)ar,  labradorite.  he 
also  frecjuently  mentions  the  lime  felds])ar.  anorthite,  as  an 
essential  and  predominant  ingredient.  He  allows  the  name 
diabase  sometimes  to  apply  to  a  I'ock  containing?  anorthite.  but 
when  dialla«.'e  a])pears  with  the  usual  as.sociates,  such  as  a 
pla^^ioclase-feldspar.  titaniferous  ma<?netito  and  perhaps  biotite 
and  augite.  the  rock  is  i)ut  in  his  third  class  under  "gabbro". 
In  thus  adhering  to  the  presence  or  absence  of  diallage  in  the 
rock  as  the  principal  criterion  for  its  name  in  his  series,  he 
followed  the  principle  and  the  practice  of  most  of  those  lithol- 
ogists  who  had  then  studied  these  rocks. 

Later  in  the  same  volume  are  several  other  re))orts  on  these 
rocks.  Prof.  Irving's  appears  on  pp.  168  83.  He  places  the 
Bad  river  gabbro  belt  at  the  base  of  the  Keweenawan  series, 
and  makes  the  important  statement  that  this  rock  "appears  to 
alternate  with  the  ordinary  diabase".  If  there  be  no  mistake  in 
this  observation  it  will  be  found  to  go  along  way  toward  estab- 
lishing the  idea  that  the  gabbro  belt  is  linked  inseparably  with 
the  diabases  of  the  Keweenawan.  It  does  not  appear,  however, 
that  the  details  of  this  alternation  have  been  given  by  Irving,  nor 
by  any  other  observer.  It  may  be  that  Irving  refers  here  not  so 
much  to  a  structural  interbedding  as  to  a  geographical  succes- 
sion of  belts  or  areas.  From  such  a  succession,  /.  e.  one  of 
geographical  alternation,  an  observer  with  Prof.  Irving's  pre- 
conceived notion  of  the  relation  of  the  gabbro  to  the  Keweena- 


»  Lltliology  of  tilt  Keweenawan  system.    Wisconsin  Report,  Vol.  Ill,  p.  20,  1880. 


THK   NOUIAN    OF   THK    NOUTII WKST. 


a 


wan,  and  tin-  itit<'V<,''railati(»ti  of  its  roclcs  |H>tr()<,'i"i|)lii(ally  with 
thosi'  of  llic  K«'W<MMiawaii.  would  almost  iin'\ital)ly  infer  a 
Ktruclui'al  intorboddin^.  Y«'l  it  is  ([uitt'  probaljlc  that  \\w  iil 
tortiation  was  a  ^«!o^ra|)hical  oiu',  ind  hence  indicates  an  iiii 
confoiMiiahle  overlying'  ot  Keweenawan  s\irfiic«!  Hows  on  a  series 
of  rid^'es  of  ^abl»ro,  followed  l)y  abrasion  and  plowinfjf  such  as 
tliat  produced  by  the  (ilacial  epoch.*  In  this  <;aljbi'o  Ijelt.  ac- 
cordiii',''  to  l*rof.  lrvinj?"s  descriptions  there  are  two  varieties 
of  j^abbro.  viz:  1st.  -  That  which  lies  farthest  south,  a  bluish- 
^ray  to  nearly  black,  highly  crystalline  rock,  varying  in  text- 
ure from  very  tine  to  vei-y  coai'se  "rrained  when  the  individual 
crystals  reach  two  or  three  inches  in  len^'th.  The  normal  con- 
stituents are  coniinoidy  labradorite.  ••auirite  or  dialla^'e". 
magnetite  or  titanic  iron,  and  olivine,  the  last  not  bein<f  invari- 
ably i)resent.  Hiotite  is  also  common.  L'nd.  In  the  mon* 
northern  porti(»ns  of  the  principal  «;al)bro  belt  are  found  sub 
ordinate  belts  of  a  rock  which  is  red  and  black'  mottled,  or  red- 
black-aud-<,'ray  mottled,  coarse-grained  though  never  so  coarse 
as  the  other  variety ,  and  marked  by  a  gnuiter  amount  of  titanic 
iron.  The  ingredients  of  this  rock  are  the  same  as  tliose  of 
the  other  variety,  except  that  olivintj  is  only  rarely  pi-eseni. 
and  is  then  apt  to  be  changed  to  a  greenish  mineral,  and  the 
plagioclase  is  sometimes  brick  red.  ••The  augitic  ingredient 
in  this  roclf  is  always  higlily  foliated. "t  and  in  some  cases  it  is 
uralitized  and  even  converted  to  a  greenish  chloritio  substance. 
After  a  somewhat  gen<!rali/ied  microscoi)icaldt»scriptiotiof  each 
variety  I'rof.  Irving  concludes  that  his  ol)servations  bear  out 
the  conclusions  of  certain  European  lithologisls  as  to  lh(>  sub- 
ordinate importance  of  the  foliated  condition  of  augite.  ••by 
which  gabbro  is  ordinarily  separated  from  diabase,  of  which  it 
would  seem  to  be  merely  a  phase".  Yet  Irving  retained  the 
name,  both  in  his  descriptive  text  and  in  his  maps,  "not  only 
because  most  of  our  rock  is  very  close  to  the  typical  Euroi)ean 
gabbros,  but  more  especially  i)ecause  it  is  so  sharply  contrasted 
with  the  typical  Keweenawan  diabase  that  a  separate  name 
seems  necessary."  He  found  no  indication  of  bedding  like  that 
seen  in  the  diabase.  He  found  tln>  gabbro  cut  by  intrusive 
granite,  much  of  it  being  tine-grained  and  pinkish  mixtures  of 
orthoclase  and  quartz. 


*  There  Is  a  u'ciioral  rlcscriptloii  (if  tills  •altcniiitldn"  cm  pp.  I'iT— 7S.  Vol.  III.  flroloify 
of  Wlsc-onsin,  wliete  it  appeiirs  Dial  I tiere  was  ohsi-rveil  iiotlilii;;  more  tliuii  an  inter- 
rupted series  of  exposures  of  iiabbri).  with  diabase  iiilerveiiliii:. 

<   Irviui;,  Geology  ot  Wlseonslii,  Vol.  Ill,  p.  ITI. 


VI 


BULLETIN    NO.    VJH. 


Irving  put  lln'  j^abbro  in  Um?  Kowoonawan  for  tlio  following 
reasons.     ((Jool.  Wis.  Vol.  III.  i)p.  171   171'). 

1st.  -  Th«'  clo.so  similarity  it  pni.sonts  in  min»»ral  coinpo.sition 
to  tho  truo  K«'\v(M»na\van  diuba.s'^. 

-nd.  The  cvidont  intorstratitioation  with  the  latter  noar  tho 
.nmction  of  tlio  two. 

;ir<l.  Th(j  inaiinor  in  which  iininistukabh?  Kowocnawan  dia- 
base tills  tho  oast(»rn  i>xtonsion  of  tin'  j,'ubl)ro  bolt  in  the  vicinity 
of  Montreal  river. 

1th.  The  niassivenoss  and  apparent  eruptive  nature  of  the 
^abbro. 

'ith.  Tlie  occurrence  of  «,'abbros  in  tho  typical  Keweonawan 
lejjfion  of  K(;we(»naw  point,  and  with  tho  typical  Ivewoenawan 
diubasc!  in  Doii^'las  county,  Wisconsin. 

(ith. — The  appai'ent  non-conformity  of  tho  j^abbro  and  the 
Huronian  schists,  as  indicated  by  the  vvay  in  which  the  junc- 
tion lint!  l)etween  the  two  cuts  diagonally  across  the  strike  of 
the  Huronian  Ijods. 

In  tho  description  of  these  «?al)bros  both  labradorite  and  an- 
orthito  are  m<Miti()ne(l  as  constitutin«r  the  feldspar  ingredient, 
and  the  augite  is  sonietimcvs  laminated  and  sometimes  n(>arly 
unchang<'d.  One  tield  exposuri?  is  described  as  "light-gray, 
often  nearly  w\iite  gabbro,  a  peculiar  phase  not  noted  else- 
where. Under  the  micro.scope  the  largo  i)lagioclasos  make  up 
nearly  tho  whole  section."  Indeed  both  lie  and  IMuupelly  refer 
in  several  instances  to  the  exceeding  coarseness  of  the  plagi- 
oclase  ci'ystals.  and  the  great  })reponderanco  of  the  feldspar 
over  all  tlu;  other  constituents  of  the  rock. 

The  third  annual  report  of  the  director  of  tho  United  States 
geological  survey  contains  a  somewhat  later  ( 1HM3)  exposition 
of  th(!  Keweonawan  by  Prof.  Irving,  embracing  some  now  field 
observations  and  some  new  lilhological  studies.  Here  ho  still 
retains  tho  gabbro  series,  whether  in  Wisconsin  or  in  Minne- 
sota, in  the  Keweenawan;  announces  a  new  variety  of  tho  gab- 
bro series  from  Minnesota,  viz.:  "anorthite  rock."  which  he 
says  is  "mei'ely  a  coarse  gabbro.  in  which  all  the  ingredients 
but  the  feldspar  are  wanting."  (p.  07)  and  gives  some  of  the  de- 
tails of  (di  iinc()>if(ir)iiitij  oj  the  ffabltro  sefies  Ijelow  the  diabases  of 
the  rest  of  the  formation,  in  the  Bad  river  region.  This  jJassage 
is  so  imjwrtant  that  it  is  worth  quoting  in  full: 

"On  Bad  river,  eighteen  miles  southwest  of  the  Montreal,  the 
lower  division  [/.  e.  the  bedded  diabases]  has  a  surface  width, 
from  the  Huronian  slates  below  to  the  sandstones  of  the  upper 


Ik; 


THE   NORIAN   OF  THE    NOKTHWKKT. 


VII 


division,  of  only  17,000  foot.  Sinco  tlio  (lip  horo  is  porpondic- 
ular,  or  nearly  so,  the  thicknoHS  is  not  much  Iosh  than  this.  It 
is  shown,  in  the  ori^final  nionioir  [Monograph  V,  U.  S.  Gool. 
Surv«;y|.  that  this  oxtraordiiiary  thinniri«r  is  coriiioctod  with  the 
proscnco  ht'low  of  a  ^roat  holt  of  tiio  coar.so  <;al)l)ro  dos(',i"il)od 
in  a  procodin^  pai'a^rraph  of  this  cliaiitor.  Tliis  ooarso  <.,'al)l)ro 
— whethorwith  or  without  int(ul)od(U'!d  tine-Ki"vinod  bods  is  not 
now  Ifnown—  usurps  most  of  tho  thicknoss.  ioaving  only  some 
5,000  f(»ot  for  tho  usual  thin  boddod  flows  of  tho  lowor  division. 
Tho  oxplaiiation  may  Ijo  that,  oarly  in  tl\o  history  of  tho  sori»!s 
theri!  was  ]>ourod  out  horo  an  immonst;  thiclcnoss  of  a  rock 
which  soliditiod  into  the  coarse  j^'aijhro.  whilo  later  in  its 
f,'rowth  tho  vonts  were  removed  from  here  to  either  side.  The 
coarse  gabbro  mass  must  have  stood  up  to  a  groat  hight,  and 
tho  later  flows  toi-minatod  against  it  on  oithor  sidc^  until  they 
had  accumulated  sutticiontly  to  overflow  its  upper  surfacu," 
(pp.  130-7).  On  page  HM,  also.  Prof.  Irving  si)oaks  of  those 
coarse  gray  gabbros.  stating  that  they  "present  the  appear 
ance  of  a  certain  sort  of  unconformity  with  the  overlying 
bods.'" 

It  a])pears  from  this  that  profossor  li'ving  was  not  willing  to 
state  un([ualitiodly  that  tlie  galibro  and  tla;  ilial)ase  were  inter- 
bedded  in  tho  Bad  river  region.  It  also  app(>ars  that  in  the 
same  region  the  same  relation  of  nonconformity  obtains  as  in 
Minnesota,  although  its  significance  was  not  fully  entertained 
by  Irving.  Thus  Irving  saw  some?  of  tho  evidences  adduced 
by  Dr.  Lawson  lor  considering  the  gabbro  n^cks  ol"  Wisconsin 
and  Minnesota  as  constituting  a  formation  intermediate  be- 
tween the  diabase  phase  of  the  Kewoonawan  and  the  formation 
which  he  styled  Huronian,  which  latter  is  a  far  older  terrano. 
b(,'ing,  undouljtodly.  one  of  tho  series  which  we  now  know  as 
Ontarian.  and  probably  the  lower.  It  was  owing  to  Prof.  Irv- 
ing's  confusit)n  of  rocks  of  several  distinct  horizons  under  the 
general  term  Huronian  that  he  fell  into  the  error  of  putting  the 
gabbro  into  the  Keweenawan,  an  error  that  has  vitiated  the 
work  of  some  of  his  as.sociatos.  and  that  of  some  of  his  follow- 
ers, and  wiiich  projected  itself  into  the  geology  of  Minnesota. 
Whereas  he  saw  the  gabbros  overlying  and  traversing  the 
strike  of  the  ••Huronian"  (Ontarian)  in  the  region  west  from 
Bad  river  Wisconsin,  he  inferred,  they  must  be  later  in  date. 
Again,  when  he  found  the  '•Huronian"(Animike)  associated  with 
rock  "identical  with  the  Duluth  gabbro."  on  Pigeon  point,  in  a 
manner  not  readily  determined,  he  applied  the  conclusions  of 


mm 


iv 


VIII 


BULLETIN    NO.    VIII. 


the  former  obsor\'ation,  and  was  moved  to  place  the  gabbro 
above  the  Animikio,  at  the  base  of  the  Kevveenawan,  although 
there  is  at  that  point  no  evidence  whatever  looking  in  that  di- 
rection. Had  he  understood  how  much  lower  in  the  strati- 
graphic  scale  the  Bad  river  "Huronian"  is  than  the  "Huronian" 
with  which  he  compared  it  in  other  parts  of  Wisconsin  and  in 
Minnesota,  he  would  probal)l3'  have  given  due  heed  to  both  of 
the  \inconformities  which  he  briefly  notes,  and  would  have  re- 
tained the  gabbro  in  that  stratigraphic  position,  where  for 
some  reasons  he  had  felt  inclined  to  put  it. 

It  is.  perhaps,  due  largely  to  the  researches  of  professor 
Irving  that  the  appli'3ation  of  the  term  gabbro  in  the 
Northwest  was  extended  beyond  the  rocks  that  possess  the 
petrographic  characters  at  first  included  under  it  by  Kloos  and 
Streng,  and  by  Pumpelly.  Irving  found  all  conditions  of 
change  in  the  augite  toward  diallage,  and  all  conditions  from 
diallage  to  chlorite.*  The  chief  distinction  between  the  gabbro 
and  the  diabases  of  the  "lower  division"  of  his  Keweenawan 
consisted  in  outward  structural  features,  and  in  visible  petro- 
graphic characters.  The  gabbro  is  anbedded;  the  diabases 
always  evidently  bedded.  The  gabbro  is  generally  light-colored 
and  coarse-grained,  never  amygdaloidal,  the  diaba.ses  are  dark, 
fine-grained  and  frequently  amygdaloidal.  Although  he  main- 
tained that  the  gabbro  was  extravasated  as  a  surface  flow,  and 
"cannot  have  been  intrusive",  yet  he  noted  many  of  the  evi 
dences  that  disagree  with  that  view.  In  short,  structurally 
and  petrographically  the  gabbro,  in  all  respects.was  thought  to 
grade  into  the  Keweenawan.  This  view  was  announced  in  his 
Wi-sconsin  reports,  and  his  later  work  for  the  United  States 
Geological  Survey  does  not  vary  in  any  of  its  main  results, 
from  the  conclusions  of  the  Wisconsin  reports. 

When  we  look  into  the  reports  of  the  Minnesota  survey  we 
find  the  term  gabbro  applied  to  the  Duluth  rock  first  in  187!) 
(published  in  1880),  at  page  23.  The  present  Avriter  began  a 
preliminary  microscopical  examination  of  a  series  of  rocks  col- 
lected by  the  survey,  mainly  along  the  shore  of  lake  Superior, 
and  in  August,  1881,  he  read  a  paper  before  the  American 
Association  for  the  Advancement  of  Science,  t  in  which  this 
rock  is  referred  to,  but  is  included  in  the  Cupriferous  series. 
This  was  based  on  field  examinations  conducted  in  1878  and 
1879. 


*  Third  iiiiniial  report  of  tlic  director  of  tlic  I'nlted  State  OooloKlcal  Survey,  p.  10). 
+  Clneiniiiitl  meeting,  p.  IfiO.    Tenth  Annual  Report.  Minnesota  Survey,  p,  t37. 


THE   NORIAN    OF  THE   NOHTHWEST. 


IX 


As  noted  by  Dr.  Lawson  fp.  14)  the  gabbro  and  feldspar  rock 
at  Beaver  bay  and  about,  Split-rock  poi  it  were  referred,  con 
trary  to  the  opinion  of  Norwood,  to  a  date  older  than  th(»  traps 
forming  the  shore.  "Large  masses  of  feldspar  rock,  (embraced 
in  the  trap,  as  boulders  are  embraced  in  hardpan  clay,  have 
been  carried  from  Carlton's  peak,  or  from  a  range  of  hills 
north  and  west  of  it.  toward  the  east  and  southeast.  These 
embraced  pieces  become  smaller  in  going  from  their  place  of 
origin,  in  the  same  manner  as  fragment  5  of  rock  acted  on  by 
the  drift  forces."  (  Seventh  Minnesotfu  rei)ort,  pp.  lL^13).  It 
was  stated  that  the  "feldspar  rock"  and  the  gabbi'o  at  Duluth 
are  of  one  and  the  same  formation,  this  statement  being  based 
on  the  existence,  in  minute  quantities,  not  only  of  all  the  gab- 
bro  ingredients  in  even  the  feldspar  masses,  but  the  existence 
in  the  pudding-stones  where  the  feldspar  masses  prevail  of 
some  rounded  gabbro  masses  showing  various  degrees  of  trans- 
ition to  pure  feld.spar.  the  augite  and  magnetite  being  evident 
to  the  unaided  eye,  the  former  frequently  chloritic.  The 
writer  had  also  examined  the  gabbro  range  at  points  remote 
from  the  coast  line,  and  had  expressed  a  similar  conclusion. 
The  following  is  from  the  tenth  annual  report,  p.  80: 

"The  mineral  composition,  however,  is  constant,  and  allies 


it  to  the 


Igneous 


rocks  of  the  'gabbro  range"  which  graduate 


into  the  trap  rocks  of  the  Cupriferous.  It  seems,  however, 
that  there  was  a  vast  outflow  of  igneous  rock  in  the  midst  of 
the  era  of  the  quartzyte  and  slate  group  producing  this  area  of 
the  gabbro  range,  and  separating  the  lower  portion  from  the 
era  of  the  porphyries  and  the  red  shale  and  the  red  sandstone 
which  are  characteristic  of  the  Cu])i-iferous  formation  proper. 
There  are  some  reasons  for  believing  that  this  great  igneous 
outflow  entered  the  sedimentaries  as  laccolites  in  many  places 
and  thus  tilted  and  modified  the  overlying  beds,  instead  of 
being  produced  prior  to  their  deposition." 

Again  on  page  99.  tenth  report: 

"On  the  south  side  of  Little  Saganaga  lake  the  rock  weath- 
ers white.  It  rises  in  higher  bluffs,  resembling  the  Rice  point 
gabbro,  and  even  approaching  the  whiteness  and  nearly  the 
purity  of  the  so-called  'feldspar  rock'.  It  is  mainly  of  feldspar, 
but  also  contains  magnetite  and  a  little  pyroxene.  The  hills 
and  ridges  in  general  show  a  coarse  bedding  which  dips  south. 
Ridges  10  to  50  f(»et  high.     The  sample  collected  is  weathered. 

*        *        *        *        A  pinkish-red  vein  of  syenite,  20  inches 
wide,  cuts  this  gabbro.     It  is  comparable  to  the   red  syenite 


X 


BULLETIN   NO,    VIII. 


associated  with  the  <?abbro  at  Rice's  point.  In  both  places  it 
penetrates  irregularly  through  the  gabbro.  *  *  *  * 
This  formation  not  only  seems  to  be  the  "Rice  Point  granite', 
and  the  iron-bearing  rock  of  Mayhew  lake,  but  also  to  have 
furnished  ihe  feldspar  masses  of  Castle  Danger  and  Beaver 
bay." 

After  an  interv^al  of  several  years  during  which  these  rocks 
were  not  studied  by  the  Minnesota  survey,  but  during  which 
the  last  results  of  Prof.  Irving's  work  on  them  were  published 
by  the  United  States  Geological  Survey,  the  attention  of  the 
Minnesota  geologists  was  again  directed  to  them.  While  Prof. 
Irving's  large  volume  (Monograph  v.  of  the  U.  S.  Geological 
Survey)  serves  as  a  valuable  systematic  presentation  of  the 
geology  of  the  "Keweenawan"  as  known  to  him  through  his 
studies  for  the  Wisconsin  survey,  much  of  it  greatly  amplified 
by  more  extensive  travels,  and  more  extended  petrographical 
study,  yet  its  conclusions  do  not  depart,  in  any  important  re- 
spect, from  those  published  several  years  earlier  by  the  state 
surve3^  He  admits  himself  that  he  was  only  able  to  add  two 
new  varieties  of  rock  worthy  of  names  to  the  classification 
adopted  before,  viz:  anorthite-rock  and  diabase  porphyryte, 
and,  near  the  close  of  his  investigations,  resuming  the  exam- 
ination of  the  former,  evidently  after  the  volume  had  been 
printed,  he  found  that  the  feldspar  from  which  the  name  of  the 
rock  had  been  taken,  "does  not  correspond  in  composition  to 
typical  auorthite,"  but  rather  resembles  that  of  labradorite.* 
It  soon  became  evident  that  there  w^as  something  yet  unknown, 
relative  to  the  gabbro  belt,  in  respect  to  its  date  and  its  strati- 
graphic  relations  with  the  rocks  with  which  it  was  most  fre- 
quently associated.  It  has  been  a  standing  problem  that  has 
jiresented  itself  obtrusively  before  us  for  about  eight  years. 
Owing  to  the  discovery  of  gabbro  interbedded  with  the  Pewa- 
bic  quartzyte  in  1887  (IBtli  report,  p.  85),  and  alternating  with 
it  in  great  belts  running  east  and  west,  having  a  common  dip 
and  strike,!  and  inlluenced  by  the  prevalent  notion  that  the 
successive  sheets  of  the  Cupriferous  were  essentially  surface 
eruptions,  we  reached  the  inference  that  the  epoch  of  the  gab- 
bro was  that  of  the  Pewabic  quartzyte,  and  hence  that  the 
gabbro  must  be  below^  the  body  of  the  Animikie.  It  should  be 
mentioned,  also,  that  a  mistaken  idea  was  held  at  first  respect- 


*t'()pi)ei'-l)e!iriiii;  rm'ks  of  L;iUe  Siiporldr,  \mge  4H8. 

tTlils  has  iR-eti  more  fully  (It.'tuoiistratoil  since  lliuii  by  the  observatluas  of  Mr.  U.  S. 
Grjiiit. 


THE   NORIAN   OF   THE   NORTHWEST. 


XI 


ing  the  place  of  this  quartzyte  with  respect  to  the  Animikie. 
It  was  due  to  observations  made  at  and  east  of  Wauswaugoning 
ba3^  on  the  lake  Superior  shoi'e,  in  1S79,  that  this  quartzyte, 
then  styled  Wauswaugoning  quartzyte,  was  considered  to  be 
above  the  body  of  the  Animikie  slates;  when,  however,  the  Pe- 
wabic  quartzyte  was  found  to  belong  below  those  slates,  and  to 
carry  the  gabbro  with  it,  there  was  no  alternative  but  to  sep- 
arate the  gabbro  from  the  Cui)riferous  by  allowing  the  inter- 
position of  the  Animikie  between  it  and  the  diabases,  which 
characteristically  mark  the  lower  Keweenawan.  Only  remark- 
ing that  later  studies  have  tended  to  show  that  the  Wauswau- 
goning quartzyte,  with  its  slaty  quartztytes,  is  probably  on 
the  same  horizon  as  the  Pewabic  ([iiartzyte,  it  will  only  be  nee 
essary  here  to  introduce  a  quotation  from  the  Seventeenth 
Annual  Report  (p.  52 J  to  show  the  situation  in  this  stage  of 
the  inquiry. 

"Beds  of  gabbro  are  evenly  spread  with  quartzyte  strata  above 
and  below  them,  in  the  Pewabic  quartzyte  in  northeastern 
Minnesota.  In  general  the  gabbro  lies  on  the  Animikie 
(Taconic)  in  Minnesota,  but  a  favorable  observation  made  at 
Chub  (Akeley)  lake  demonstrates  that  this  quartzyte  was 
partially  deposited  over  the  Animikie  before  the  great  gabbro 
flood  occurred.  The  usual  immediate  overlie  of  the  gabbro  on 
the  beds  of  the  Taconic  is  due  to  the  fact  that  those  beds  were 
nearer  adjacent  at  the  points   of  issue  of   the   molten  rock." 

*  *  *  *  "It  has  been  difficult  to  aflirm.  until 
recently,  the  age  of  the  gabbro  outflow.  It  has  generally  been 
considered  to  have  followed  the  Animikie  (Taconic),  but  that  it 
was  later  than  the  commencement  of  the  Potsdam  (Pewabic) 
was  not  known  to  the  writer  till  ho  made  the  observations  re- 
ferred to  above,"  (p  54). 

The  idea  that  this  quartzyte  lies  above  the  Animikie  was  of 
but  short  duration.     In  the  eighteenth  annual  report  is  a  gen 
eral  discussion  of  the  "Age  of  the  Gabbro"  (pj)  43-47)  opening 
with  these  words: 

"In  attempting  to  correlate  these  observations  with  those 
that  have  been  made  and  recorded  in  previous  years,  there  is 
some  difficulty  in  fixing  the  stratigraphic  place  of  the  gabbro 
eruption.  It  was  inferred  by  the  Wisconsin  geologists  that 
the  gabbro  sheet  was  the  bottom  layer  of  the  great  Cupriferous 
formation  or  Keweenawan.  and  this  was  accepted  and  has  been 
followed  unquestioningly  by  the  writer  in  all  previous  reports 
and  discussions  of  the  crystalline  rocks  of  the  state.     The  age 


MI 


BULLETIN    NO.    VIII. 


of  the  great  Cupriferou.s  formation,  with  its  traps  and  con- 
glomerates, in  part  at  least,  has  been  established,  with  in- 
creasing evidence  and  positiveness  as  on  the  horizon  of  the 
typical  New  York  PotsJam — at  least  at  the  horizon  of  a  quartz- 
yte  which  overlies  the  Animlkie  of  the  Northwest.  Thus  the 
gabbro  sheet  was  carried  to  tlie  same  age.  When  the  gabbro 
was  found  to  be  interbedded  with  a  quartzyte  in  northeastern 
Minnesota,  and  to  lie  upon  and  overwhelm  it  (the  Pewabic 
quartzyte),  it  was  a  necessary  inference  that  that  quartzyte 
was  the  equivalent  of  the  Potsdam— or  at  least  of  the  bottom 
quartzyte  of  the  Cupriferous — and  hence  must  lie  over  the 
Animikie.  although  at  no  place  could  the  Animikie  be  seen  in" 
terposed  between  it  and  the  gneisses  of  the  Giant's  range. 
When  it  was  found  that  this  quartzyte,  which  is  the  principal 
iron  hori.:onof  the  Mesabi  range,  shows  evidence,  in  the  region 
eastward  from  Pekegama  falls,  of  passing  below  the  body  of 
the  Animikie  strata,  necessarily  carrying  with  it  the  gabbro 
sheet,  the  idea  that  the  gabbro  has  possibly  been  put  into  a 
wrong  position,  is  brought  out  prominently  before  the  student 
of  northwestern  stratigraphy,  and  he  is  disposed  to  call  in 
question  the  datum  from  which  some  important  conclusions 
have  been  drawn." 

In  brief,  the  following  considerations  then  were  summarized 
for  questioning  the  view  that  had  hitherto  been  accepted,  the 
same  being  indications  that  the  gabbro  lies  below  the  Animikie 
and  that  the  Keweenawan  series,  in  its  full  scope,  embraces  not 
only  its  sandstones  and  diabases,  so  well  known,  but  also  the 
underlying  Animikie.  This  view  was  not  a  little  strengthened 
by  the  discovery,  reported  by  Dr.  A.  C.  Lawson  *of  metallic 
copper  in  the  Ammikie  rocks  in  the  Thunder  bay  district, 
associated  with  an  amygdaloidal  dyke  of  trap. 

1.  "The  most  important  and  significant  fact  that  bears  on 
the  stratigraphic  position  of  the  gabbro,  respecting  its  rela 
tion  to  the  Animike  black  slates,  is  its  occurrence  along  a  wide 
extent,  reaching  from  Guntlint  lake  southwestward  as  far  as 
to  the  railroad  crossing  at  Mallmann's  (at  least), neat  to  and  im- 
mediately south  either  of  the  gneiss  of  the  Giant's  range  or  of 
the  "greenstones  '  of  the  Kawishiwin.  without  the  appearance 
of  any  of  the  black  slates  between  them.  There  is  an  appear- 
ance of  (juarfzyfe,  with  olivine  grains  and  with  magnetite,  geo- 
graphically between  the  gneiss  and  the  gabbro,  the  same  being 
unquestionably  the  Pewabic  quartzyte  seen  near  Gunflint  lake. 

♦American  GeoU)>?lst,  March,  1800.  vol.  v.  p.  17-t. 


, 


THE    NORIAN   OF   THE   NORTHWEST. 


XI II 


This  quartzyte  sometimes  is  impui'e  an  1  limonitio.  and  seems 
to  be  the  cliiof  iron  horizon  of  the  Mosabi  range.  This  near 
conjunction  (which  is  sometimes  apparen  ly  an  exact  contact) 
of  the  «^abbro  with  the  gneiss,  and  the  absence  of  tj^e  Animike 
proper  between  them,  has  been  supposed  to  be  due  to  a  local 
overlap  of  the  gabbro  beyond  the  strike  of  the  Animike,  cover- 
ing it  from  sight,  the  idea  being  that  the  gabbro  flowed  back 
northward  over  older  formations,  and  came  on  to  the  gneiss. 

2,  Although  there  has  not  yet  been  any  careful  microscopical 
examination  into  the  ditferences  between  the  typical  gabbro 
(for  instance  that  seen  at  Rice's  point,  near  Duluth)  and  the 
eruptive  rocks  that  overlie  the  Animike  black  slates  at  Gun- 
flint  lake  and  eastward  to  Pigeon  point,  it  has  been  noticed  that 
there  are  microscopic  distinctions  which  ought  to  be  explained 
in  case  of  a  supposed  parallelism  of  one  rock  with  the  other. 
The  supposition  has  been  that  they  are  stratigraphically  and 
chronologically  the  same,  and  that  the  differences  were  only 
local  and  imimportant.  It  was  this  assumed  parallelism  and 
the  evidently  later  age  of  the  eastern  outcro|)s  (the  "crowning 
overflow"  of  the  Animike)  which  has  led  to  the  placing  of  the 
gabbro  later  than  the  Animike.  There  is  absolutely  no  other 
evidence.  If  these  two  eruptive  rocks  are  not  contemporai'y, 
there  is  not  only  no  reason  against,  but  considerable  evidence 
in  favor  of  placing  the  typical  gabbro  (such  as  at  Rice's  point, 
and  at  Little  Saganaga  lake)  below  the  body  of  the  black  slates. 

3.  Boulders  of  characteristic  gabbro  and  of  red  syenite,  and 
of  the  quartz  porphyry,  occur  abundantly  in  the  later  "traps" 
of  the  Cujiriferous.  The  quartz-porphyry  pebbles  are  so 
abundant  as  to  constitute  the  well  known  thick  beds  of  coarse 
conglomerate;  and  quartz  porphyry  layers  or  lenticular  shceta 
are  interbedded  between  the  trap  sheets.  This  quartz-porphyry 
in  some  cases  appears  to  have  originated  in  its  present  condi 
tion  of  interleaved  sheets  during  the  time  of  the  Cupriferous. 
This  is  observable  at  the  mouth  of  Baptism  river,  and  at  the 
Great  Palisades.  At  these  points,  however,  owing  to  the  prox- 
imity of  bosses  of  gabbro  rising  above  the  rest  of  the  country 
about,  it  is  certain  that  those  portions  of  the  Cupriferous, 
which  contain  the  original  quartz-porphyry  beds,  are  near  the 
bottom  of  the  formation.  This  is  further  shown  by  the  exist- 
ance,  in  the  same  region  (at  and  near  Beaver  Bay),  of  large 
boulder-masses  of  gabbro  in  the  trap  flows,  evidently  derived 
from  the  neighboring  gabbro  hills.  Prom  this  point,  north- 
ward to  the  gneiss  of  the  Giant's  range,   nothing  is  visible,   in 


XIV 


BULLETIN   NO.    VIII. 


the  form  of  rock  in  situ,  except  gabbro,  or  some  *  "muscovado" - 
like  rock  described  at  some  outcrops  somewhat  further  west* 
by  H.  V.  Winchell.  The  region  is  not  fully  explored,  but  it 
appears  frc^m  all  that  is  known,  that  there  is  nothing  to  be 
found  of  the  typical,  thin  black  slateS  of  the  Animike.  It  is  as 
reasonable  to  infer  that  they  followed  after  the  gabbro  flood, 
as  that  they  preceded  it.  In  case  they  followed  after  it,  their 
typical  characters  were  destroyed  in  this  region  by  the  fre- 
quent outbursts  of  igneous  eruption,  and  they  blended  with  the 
tuffs  and  shales  and  basic  sheets  that  constitute,  on  the  north 
shore  of  lake  Superior,  the  lower  portion  of  the  Cupriferous 
formation.  In  case  they  preceded  they  must  exist  buried  be- 
low the  gabbro,  as  hitherto  supposed.         #        *        *        * 

5.  On  the  supposition  that  the  Animike  black  slates  are  in- 
volved in  the  Keweeuawan,  and,  while  overlying  the  gabbro, 
lose  their  typical  characters  at  points  further  southwest,  the 
interbedding  of  the  Animike  with  beds  of  trap-rock,  which  is  a 
common  feature  about  Gunttint  lake  and  on  the  shores  of  Loon 
lake,  is  easily  explained,  and  indeed  falls  into  place  as  one  of 
the  to-be  expected  facts  of  such  a  period  of  recurring  erup- 
tions. It  also  obviates  the  neci  ssity  of  a  supposed  change  in 
the  character  of  the  eruptive  rock,  i.  e.,  that  the  gabbro  of 
Rice's  point  and  Little  Saganaga  lake  becomes,  on  Pigeon 
river,  the  dark  or  greenish  trap-rock  and  the  dioryte  which  in- 
ter-bed and  characteristically  overlie  the  Animike,  forming  the 
well-known  'crowning  overflow'  of  that  region." 

Again,  in  "The  Iron  Ores  of  Minnesota,'"  there  are  some  con- 
siderations touching  the  nature  and  date  of  the  gabbro  dis- 
turbance, f 

"The  rock  itself  is  gabbro,  a  basic  eruptive,  of  gray  color 
and  generally  of  coarse  crystalline  texture.  Its  minerals  are 
labradorite,  augite,  magnetite,  biotite,  olivine.  The  relative 
amounts  of  these  minerals  undergo  great  variation.  While 
perhaps  in  no  case  will  any  of  them  be  found  entirely  wanting 
over  large  areas,  they  are  severally  sometimes  so  scarce,  while 
at  the  same  places  some  of  the  others  prevail,  that  the  rock 
takes  on  very  contrary  aspects.  When  the  labradorite  pre- 
vails, as  about  Little  Saganaga  lake,  and  Bellisima  lake,  and 
in  Carlton's  peak,  and  in  the  feldspar  masses  that  are  embraced 
in  the  dark  trap  at  Beaver  bay,  the  rock  when  fresh  is  glassy, 


•Seventeenth  report,  p.  00,91.  Samples  387  (II) . 

+The  Iron  Ores  of  Minnesota.    Bulletin  VI,    Minnesota  Geological  and  Natural  His- 
tory Survey,  1891,  p.  12a. 


THE   NORIAN    OF   THE   NORTHWEST. 


XV 


J 


gray,  and  firm,  but  on  weathering  it  becones  almost  white. 
When  the  magnetite  prevails,  as  in  the  svburbs  of  Duluth, 
about  the  southern  environs  of  Birch  lake,  at  Iron  (Mayhew) 
lake,  and  many  other  places,  the  rock  is  black  and  firm,  and 
simply  becomes  speckled  with  lighter  spots  on  weathering,  the 
spots  indicating  the  existence  of  crumbling  crystals  of  labra- 
dorite.  When  the  olivine  or  augite  or  both  prevail,  which  is 
apt  to  be  accompanied  by  the  appearance  of  crystalline  masses 
of  hornblende,  and  in  cases  of  weathering  near  the  water,  the 
rock  has  a  green,  or  dark  green  color,  the  green  tint  being  in- 
creased by  the  conversion  through  weathering  of  some  of  these 
into  serpentine,  chlorite  or  delessite.  *  *  *  It  was 
supposed  by  the  geologists  of  the  late  Wisconsin  geological 
survey  that  the  gabbro  eruption  in  the  main,  took  place  after 
the  completion  of  the  Animikie  strata,  and  that  it  formed  the 
base  of  the  Keweenawan,  fading  off  upwardly  by  a  succession 
of  traps  and  sandstones  and  with  interbeddings  of  conglomer- 
ates and  volcanic  tuffs  into  the  most  characteristic  features  of 
the  Keweenawan.  This  view  has  also  been  held  by  all  the 
Minnesota  reports  except  the  eighteenth.  But  it  has  been 
found  that  the  great  gabbro  flood  of  northeastern  Minnesota 
was  outpoured  at  an  earlier  date.  In  the  sixteenth  annual  re- 
port* will  be  found  evidence  that  it  began  during  the  deposi- 
tion of  the  Pewabic  quartzyte  *  *  *  ^  and  must  follow 
it  to  the  lower  portion  of  the  Animikie,  and  hence  to  near  the 
commencement  of  the  Taconic." 

On  the  geological  map  which  accompanies  *  'The  Iron  Ores  of 
Minnesota,"  the  gabbro  belt  is  roughly  laid  off  and  represented 
as  a  distinct  terrane  separate  from  the  diabases  and  conglome- 
rates of  the  Cupriferous  proper,  under  the  name  Norlan.  By 
an  omission  in  the  "transfer"  by  the  lithographer  the  Carlton's 
peak  area  was  not  represented  as  Norian.  although  the  Beaver 
Bay  area  of  feldspar  rock  was  included.  This  separated  part 
was  designed  to  include  also  all  the  red  syenites,  diorytes  and 
felsytes  with  which  the  gabbro  rock  is  closely  associated.  In 
thus  removing  large  masses  of  crystalline  rock,  usually  con- 
sidered to  be  portions  of  the  Archa3an  from  the  Keweenawan, 
it  was  not  intended  to  convey  the  idea  that  they  should  be 
placed  in  the  Archiean.  It  had  been  a  growing  impression, 
and  had  reached  almost  a  conviction,  resulting  from  an  attempt 
to  comjjare  and  adjust  the  results  of  study  on  these  rocks  in  Min- 
nesota with  the  published  results  of  the  Wisconsin  survey,  that 

ages  85,  88;  17th  report,  pp.  5a.  53;  18th  report,  pp.  43-17. 


XVI 


MULLKTIN    NO.    VIII. 


thoro  was  a  <rrnai  sorio.s  of  crystallino  rocks,  lator  than  the 
Ontarian  of  the  Arcliioaii.  of  which  thoro  had  not  V)een  made 
any  just  and  con-ect  hit(.'r[)retation.  Sometimes  these  roclcs 
have  been  studied  in  the  lield  in  Michi^yan  and  Wisconsin,  and 
they  had  been  jjut  eith(»r  in  the  "Huronian",  that  catch-all 
which  covers  mucli  <^('ol()^'ica]  ijjnorance,  and  sometimes  they 
had  betui  included  in  the  Ivevveenawan.  It  was  quite  evident 
also  thai  the  earlier  <?eolo^nsts  of  New  York  and  Canada  had 
included  them  in  the  Laurentian.  or  in  the  upper  Laurontian, 
sometimes  designating  them  as  • -upper  gneisses".  At  no 
place  has  it  been  ap])roi)riate  to  fully  discuss  this  question  in 
the  publications  of  the  geological  survey.  The  bare  announce- 
ment was  made  in  the  above  mentioned  maj),  because  it  was 
necessary  to  expi'ess  something  in  the  form  of  a  "classifica- 
tion", of  the  iron-bearing  rocks,  but  the  reasons  therefor  were 
not  given,  as  noted  by  Dr.  Law  son.  The  first  expre.ssion  of 
doubt  as  to  the  correctness  of  this  idea  of  their  Laurentian  age 
in  Minnesota  was  in  the  following  words,  found  in  the  ninth 
annual  report  of  the  Minnesota  survey,  p.  387.* 

"The  mineralogical  characters  of  these  belts  of  igneoasrock, 
which  form  some  of  the  main  features  of  the  topography,  seem 
to  ally  them  to  the  Norian  rocks  of  T.  S.  Hunt,  and  to  the 
labradorite  rocks  of  Canada.  At  least,  if  they  be  not  the 
western  extension  of  those  formations,  then  those  formations 
have  not  yet  been  discovered  In  Minnesota.  But  several 
traverses  have  been  made  of  the  country  northwest  of  lake 
Superior  for  the  purpose  of  geological  examinations  without 
finding  anything  that  is  at  all  comparable  to  those  formations  if 
it  be  not  the  rock  of  these  hill-ranges.  The  rock  consists  gen- 
erally, of  some  feldspar,  which  at  Duluth  has  been  found  to  be 
labradorite  in  largo  per  cent.,  and  at  some  places  constituting 
over  ninety  per  cent,  of  the  mass,  with  varying  proportions  of 
augito  or  of  magnetite,  or  magnetic  menacannite,  with  various 
accessory  ingredients,  or  of  minerals  that  result  from  change. 
It  is  massive,  firm,  usually  dark  colored,  and  rises  in  low 
mountain  ranges,  as  already  stated.         *        *        * 

So  far  as  examined  these  labradorite  rocks  contain  no  bands 
of  limestone.  In  the  absence  of  this  element,  and  in  this  only, 
so  far  as  can  be  judged  by  the  writer,  these  labradorite  rocks 
seem  to  differ  from  the  labradorite  rocks  of  the  'upper  Lau- 
rentian'of  Canada. "     These  rocks  being,  therefore,   in   Minne- 


*  Also  in  till-  Proceedin},'s  of  the  Arnerlosm  Assooiation    for  tlie  nclvanceiiient  of 
seicnce,  1880.    The  Cuitriferou»  Heries  in  yjinnexuta. 


THE    NOUIAN    OP   THE    NOKTinVKS'l 


XVII 


sota,  considerably  younger  than  the  Laurentian.  there  was 
thought  to  be  good  reason  for  suggesthig  that  the  Eozoon  cana- 
dense,  characteristic,  as  supposed,  of  the  upper  Laurentiaii  in 
Canada,  "may  be  instead  an  organism  of  Cambrian  or  lower 
Silurian." 

Notwithstanding  this  and  other  expressions  of  similar  pur- 
port, scattered  through  the  rei)orts  of  the  Minnesota  survey,  it 
was  not  until  the  publication  of  the  "Iron  Ores  of  Minnesota" 
that  any  attempt  was  made  to  give  general  oxjiression  to  a  con- 
clusion, and  in  that  case  it  was  without  basing  it  on  any  discus- 
sion or  i)utting  forward  of  the  data  on  which  it  might  be  based. 
Indeed,  whatever  the  age  of  the  Xoridn,  so-called  in  Minnesota, 
there  was  no  intention,  even  at  that  time,  to  separate  it  from 
the  formation  long  known  as  Cupriferous,  and  later  as  Xipigon. 
or  Keweenawan.  But  it  was  expected  to  retain  it  within  that 
formation  by  some  internal  reconstruction  of  its  stratigraphic 
components,  or  by  an  enlargement  of  its  stratigraphic  compass. 
It  was  suggested,  in  line  with  this  purpose,  that  perhaps  the 
Animikie  disappeared  from  view  between  Pigeon  point  and 
Beaver  bay,  because  of  a  change  in  the  characters  of  the  co- 
temporary  sedimentation,  by  the  action  of  which  its  character- 
istic slates  and  quartzytes  were  replaced  at  the  same  horizon  by 
red  shales,  tuffs,  sandstones  and  conglomerates,  such  as  prevail 
along  the  lake  Superior  shore  westward  from  Grand  Marais. 
Such  a  transition  at  the  horizon  of  the  Animikie  would  make  it 
necessary  to  include  the  Animikie  in  the  Cupriferous,  and 
would  remove  the  obstacles  which  otherwise  existed  to  the 
retention  of  the  Norian,  or  gabbro  belt,  at  a  horizon  below  the 
Animikie,  or  near  its  base.  It  is  to  the  discernment  of  Dr. 
Lawson  that  we  are  indebted  for  the  first  sutiticient  exposition 
of  the  relations  of  the  characteristic  Cupriferous  to  these  gab- 
bro rocks, put  all  together  in  a  systematic  discussion,  with  vivid 
illustrations.  The  writer  has  no  hesitation  in  adopting  the  re- 
sults of  Dr.  Lawson's  work,  for  he  regai'ds  them  not  only  as 
well  founded,  but  as  a  very  welcome  solution  to  a  very  per- 
plexing series  of  facts  which  needed  and  had  not  yet  received 
any  satisfactory  adjustment. 

There  remain,  however,  one  or  two  minor  points  which  the 
careful  reader  will  note, which  need  further  consideration, inas- 
much as  this  interpretation  of  Dr.  Lawson's  report  carries  its 
application  to  an  extent  somewhat  further  than  he  has  himself 


, 


XVIII 


lUJLLKTIN    NO.    VIII. 


indicated,  tho  important  si^niticanco  of  which  he  may  not  have 
foreseen,  or  may  not  have  been  willing  to  acc(3j)t  if  for(3soen. 
These  are: 

1.  Is  the  anorth<)syt(!  described  Ijy  Dr.  Lawsoii  at  Carlton 
peak,  Beaver  bay  and  Split- roclc  point,  j^ooKi"i|)liically  a  por- 
tion of  the  great  gabbro  belt,  and  identical  with  it  in  origin 
and  date?  This  hinges  very  largely  on  the  next. 

2.  Is  tiie  "anorthite  rock'  wiiich  churactorizes  tho  anortho- 
syte  at  Carlton  peak  and  Splilrock  point  p(itr()gra[)liically  the 
same  as  the  gabbro  of  the  gabbro  belt,  only  dilloring  from  it  in 
non-essential  characters? 

The  summary  results  attained  by  Dr.  Lawson  are  made  to 
apply  by  him  only  to  tho  distinctive  outcrops  at  Carlton  peak, 
Beaver  bay  and  Split-rock  jwint.  Tho  region  of  tho  Duluth 
rock,  where  the  gabbro  of  the  Northwest  was  tirst  known,  and 
has  been  most  studied,  is  omitted  by  him  from  his  discussion. 
The  same  is  true  of  points  farther  east,  such  as  the  gaobro  of 
Hat  point  and  of  Pigeon  point.  He  does  not  allude  to  outcrops 
in  Wisconsin.  His  determinations  require  a  rock  made  up 
entirely,  or  almost  entirely,  of  anorthite  or  labradorite  feldspar. 
The  specimens  he  examined  appear  to  have  been  such.  Prob- 
ably more  fleld-work  may  be  required  to  remore  all  doubt  as  to 
the  continuity  of  the  anorthite  rock  and  the  gabbi'o  geograph- 
ically, and  more  laboratory  work  to  determine  the  petrographic 
characters  that  may  ally  or  dissociate  these  rocks,  but  it  may 
be  well  to  summarize  briefly  the  facts  which  seem  at  present  to 
warrant  the  assumption  that  they  are  the  same. 

1.  They  have  been  described  by  professors  Pumpelly  and 
Irving,  as  can  be  seen  by  the  foregoing  quotations  from  their 
reports,  in  such  terms  that  they  cannot  be  separated  into  two 
formations  either  geographically  or  petrographically.  In  numer- 
ous instances  they  have  described  the  gabbro,  in  its  large  and 
characteristic  exposures  in  Wisconsin,  as  made  up  almost  wholly 
of  one  feldspar,  usually  labradorite.  and  sometimes  in  very 
large  grains  or  crystals. 

2.  When  the  present  writer  collected  samples  from  the 
boulder-masses  of  feldspar  I'ock  embraced  in  diabase  near 
Split  rock  point,  he  was  particular  to  note  that  some  of  the 
masses  were  not  of  pure  feldspar  rock,  but  contained,  in  the 
usual  proportions,  the  minerals  augite  and  magnetite.  Thus 
he  noted  in  the  field,  and  subsequently  published  the  alliance 
of  these  two  rocks  petrographically,  the  two  embraced  together 
heterogeneously  in  a  sort  of  i3udding-stone  of  trap-rock,  to  the 


THE   NOttlAN   OF   THK   NOKTHWHST. 


MX 


oxcliisioii  of  cvnry  otluM*  kind  (tl"  rock,  except  llie  red  j^ninite.* 
Sin(!(!  that  tiiiio  microscopical  lliiti  sections  luivo  reveaUnl  in  tlie 
feldspar  boulders.  howev(»r  pnro  thoy  may  appear  to  the  eye. 
small  ([uantltiesof  au<?ite,and  from  those  minute  quantities  there 
are  all  >?radations  to  tyjjically  constituttfd  <,'al)bro.  Tliere  are 
also,  am(»n«^st  th(>se  detached  masses,  some  tiiat  are  darker 
than  the  ordinary  gal)l)ro,  containing  very  lari,'e  ))roportions 
of  augite  and  magnetite,  with  some  olivine  J  It  thus  appears 
tliat  at  the  Iocks,  where  the  later  diabase  sheets  received  the 
distinctiv*^  anorthite  rock,  there  were  variations  to  typical  /^ab- 
bro,  and  even  to  the  very  dark  and  heavy  <;abbro.  S\ich  vari- 
ations have  been  noted  repeatedly  at  p<»inls  where  the  f,'abl>r() 
belt  appears  in  Wisconsin  and  in  Minnesota.  Aj?ain  the  red 
granite  i)ieces,  which  appear  as  isolated  bould(n's  in  the  diabase 
tlows  at  Beaver  bay,  indicate  the  same  relation,  for  red  ^'ranite. 
the  augito  syenyte  of  Prof.  Irving,  is  a  fre(|uent  att»'ndant  of 
the  gabbro  range  in  Minnesota,  as  well  as  in  Wisconsin. 

The  nicer  distinctions  between  the  feldspars,  i.  e.  whether 
hibradorite  or  anorthite,  do  not  seem  to  play  any  im])ortant 
part  in  the  solution  of  the  question  at  issue.  There  is  good 
authority  for  the  statement  that  both  those  feldspars  are 
found  both  in  the  anorthite  rock  and  in  the  gabbro.  al- 
though it  now  appears  that  labradorite  predominates  in  both. 
This  itself  is  a  bond  of  petrographic  alliance  between  them  and 
is  perhaps  as  strong  as  any. 

Negatively  considered,  if  the  anorthite  rock  be  not  the 
same  formation  as  the  gabbro,  room  must  bo  made  for  both  of 
them  in  the  geologic  composition  of  the  northern  part  of  the 
state,  and  it  will  require  a  greater  reconsideration  and  a  great- 
er reconstruction  to  accommodate  two  such  formations  than  to 
accommodate  one  and  implies  greater  error  in  all  previous 
work. 

The  consequences  of  this  important  result  are  far-reaching. 
It  makes  it  necessary  to  reconsider  some  of  the  descriptions 
that  have  been  published  of  the  relations  of  this  rock  to  other 
formations,  in  Wisconsin,  and  also  to  make  some  comparisons 
with  similar  geology  in  regions  further  east. 

The  Bohemian  mountains,  of  Keweenaw  point,  were  described 
by  Poster  &  Whitney]:  as  constituting  the  axis  of  an  anticlinal 


♦See  pp.  ni,  ns  and  IIU.  Tenth  Annual  Report. 

+0ne  of  the  samples  collected  l)y  the  writer,  numbered  818,  shows  this  character. 
Anotlier  of  the  same  number  is  nearly  pure  feldspar,  but  Us  grain  Is  not  so  fresh  and 
coarse  as  mucli  of  titat  seen  at  Beaver  bay. 

iUeport  on  the  Geology  and  Topottrapliy  of  a  portion  of  the  lal{0  Superior  land  dis- 
trict. I'art  1.    Copper  lands,  p.  64,  1850, 


XX 


nuLi.irriN  NO.   viii. 


I 


Htructurn,  which  thoy  gave  to  tlio  poiut.  The  rock  of  this 
r:inj;<'  wiis  said  to  he  difTcrcnl  from  that  in  tlio  iiorthiM'u  trup 
raii^fcs,  l)otli  in  litliolo^ical  cliaractci"  and  in  tho  luoch' of  its 
occurrence.  Tiic  northern  ran^i!  was  nuide  up  of  numerous 
trap  V^t^ds,  with  amyj^daloidal  and  ^'ranuUir  varieties  intorstru- 
titled,  ami  with  various  dotrital  rodcs.  especially  con^iomei-ates. 
The  Bohemian,  or  soutiiern  ran^je.  consisted  of  a  vast  crystal- 
line mass,  forniinf;  an  anticlinal  axis,  "tlank'cd  on  the  north 
by  tiie  l)edded  trap  and  con^'hjmorale,  and  on  the  south  by  con- 
glomerate and  sandstone."  The  contour  of  the  bedded  trap 
was  observed  to  be  very  dilTei'ent  I'roni  that  of  the  unVjedded. 
The  hills  composed  of  the  former  rise  by  a  succession  of  stair- 
like  ascents;  those  of  the  latter  are  either  dome-shaped  or 
rounded.  The  i-ock  itself  sometimes  (-ontains  numerous  crys- 
tals of  maj^netic  ii'ou  ore  which  occasionally  torms  a  lar^e  por- 
tion of  the  rock.  The  diajjfi-ammatic  section  given  by  Foster  & 
Whitney  (p,  00)  shows  Bohemian  mount,  "comi)osed  of  labra- 
dor  and  chlorite,"  with  its  irr(>f;ular  patches  of  mai^netlte, 
forming  a  central  range  in  the  Uivm  of  an  anticline  from  ic/iich 
lilt'  (itlicr  rocks  'm  intcnn/nniKih/t  on'riic,  dip  in  o])posite  directions. 
Numerous  geologists,  including  Jackson,  Foster  and  Whitney, 
Whittlesciy,  T.  S.  Hunt,  Gaujot,  described  the  structure  as  anti- 
clinal. ltapi)ears  that  this  structure  is  very  similar  to  that  found 
to  obtain  in  Wisconsin  and  Minnesota,  whei'i'ver  these  terranes 
exhibit  their  mutual  relations,  and  it  provokes  tlie  in(|uiry 
whether  it  may  not  be  well  to  reexamine  tlie  geology  of  Ke- 
weenaw point  with  this  (piestion  in  view.  Prof.  Irving  included 
these  rocks  all  in  on(». series,  substantially  conformable  amongst 
themselves,  under  the  term  Keweonawan,  but  it  appears  more 
than  probable  that  there  is  a  profound  break  between  the  rocks 
of  the  Bohemian  range  and  the  well  Icnown  traps  of  the  north- 
ern range,  and  that  too  mucli  was  embraced  under  the  name 
Keweenawan. 

THE   NORIAN. 

The  foregoing  descriptions  lead  irresistibly  to  a  search  for  a 
suitable  name  for  the  lower  series.  Precedent  and  scientific 
nomenclature  seem  to  require  the  use  of  an  old  name  if  it  be 
found  that  one  has  already  been  given  to  this  group  of  rocks. 
Fortunately  the  dominant  characters  of  this  group  are  very 
marked,  and  they  are  easily  found  when  they  have  been  ascribed 
to  any  other  locality.  As  we  approach  nearer  a  grand  classi- 
fication of  the  crystaline  terranes,  such  as  can  be  said  to  be  ap- 
plicable over  large  if  not  continental    areas,  it  will  become 


THE   NUlilAN   «)K   T  IK   NUUTHWKST. 


XXI 


necessary  to  abandon  sonic  of  the  local  natnos  which  have  benn 
nsod  (Inrin^r  the  coui'sc  of  invcstijfation.  and  to  clioosc  from  the 
various  synonyms  those  which  will  l)c  wai-ranlcd  i)y  the  rules  of 
^('olo^ical  nomcnclaturci.  Mr.  I^awson  has  proi)os«jd  a  n«'W 
name  (Carltonian)  and  that  will  bo  convenient  for  those  who  do 
not  yet  feel  ready  to  accept  the  ^'lander  parallelisms,  and  who 
do  not  wish  to  confound  their  studies  by  the  use  of  possildy 
aml)i^'\ious  terms.  Hut  to  the  wi'itei-  thi>  ti«'ld  study  and  the 
reseai'ch  have  been  pursued  far  enou«^h  to  warrant  the  refer 
enco  of  these  rocks  to  a  nico^ni/ed  epochal  position,  with  a 
welllfnown  desififnation.  At  tirst  calhnl  upper  L.iurentian.  by 
Logan,  because  they  \\(}  niiconformablt>  on  the  i-oal  Fjaurentian 
rocks,  north  from  Montreal;  sul)S('(|uently  rjubradoi-iaii.  by  T. 
Stei-ry  Hunt,  because  of  the  prevalence  of  that  mineral  ( lal)ru- 
dorite)  which  received  its  name  from  Labrador;  they  were 
lastly  designated,  by  the  same  <jfeolo<;ist.  Norian.  becau.se  of 
their  prevalcMice  at  Esrnark,  Norway,  where  tlie  norytos  were 
first  described.  The  term  "upper  LaunMitian"  is  objection:!, 
ble  l)ecause  there  is  rea.son  to  believe  these  rocks  are  separated 
from  the  Laurentian  by  a  long  interval  of  time,  and  because 
their  dominant  characters  show  them  to  have  been  largely  of  a 
basic  eruptive,  and  thus  very  markedly  distinct  in  date  and 
origin  fi'om  the  acidic  granites  and  gn(Msses  that  characterize 
the  pro|)(>r  Laurentian.  Were  it  not  for  this  th(i  term  ••upi)er 
Laurentian."  having  precedence  in  date,  might  be  perpetuated. 
Dr.  Hunt  was  the  author  of  both  the  other  terms,  and  his  later 
choice  was  Norian.*  His  succinct  description  of  them  is  in  the 
following  terms: 

"The  typical  norytes  consist  cliielly  of  atriclinic  feldspar, 
varying  in  comi)osition  from  anoi"tnite  to  andesine.  but  gener- 
ally near  labradorite  in  comi^osition.  The  color  of  tliese  rocks 
is  ordinarily  some  shade  of  blue,— from  bluish-black  or  violet 
to  bluish  gray,  smoke  gray  or  lavender,  more  i-arely  ])assing 
into  liesh-red.  and  occasionally  greenish-blue,  greenish,  or 
bluish-white.  The  weathered  surfaces  are  opaque  white. 
These  norytes  are  sometimes  nearly  pure  feldspar,  but  often 
include  small  portions  of  hypersthene.  pyroxene,  or  horn- 
blende,--the  former  two  being  sometimes  associated  in  the 
same  specimen  and  in  contact  with  each  other.  A  black  mitja 
(biotite),  red  garnet,  epidote,  chrysolite,  and  menacannito 
(titanic  iron)  are   frequently  present  in  the.se  rocks;  rpiartz. 


•Chemical  and  Geoloj?lciil  essays,    Tliird  edilioii,  1891,  p.  t,*?**.     The  term  was  also 
ujed  by  Hunt  In  the  second  edition  p.  278. 1878. 


XXII 


BULLETIN   NO.    VIII. 


however,  is  rarely  seen,  and  then  only  in  small  quantities. 
Through  an  admixture  of  the  first  named  minerals  these  norytes 
pass  into  hyperyte,  diabase  and  dioryte.  The  norytes  vary  in 
texture,  being  sometimes  coarsely  granitoid,  and  at  other  times 
tine  grained  and  nearly  impalpable.  The  coarser  varieties 
often  present  large  cloavable  masses  showing  the  striic  char- 
acteristic of  the  polysynthetic  macles  of  the  triclinic  feldspars, 
and  sometimes  exhibit  a  fine  play  of  colors,  as  in  the  well- 
known  specimens  from  Labrador.  A  gneissic  structure  is  well 
marlcod  in  many  of  the  less  coarse  grained  varieties  of  noryte, 
and  the  lines  of  bedding  are  shown  by  the  arrrangement  of  the 
various  foreign  minerals.  Although  norytes  predominate  in 
the  Norian  series,  they  are  found  in  the  area  of  these  I'ocks 
which  is  seen  te  the  north  of  Montreal  to  be  interstratitied  with 
beds  of  micaceous  orthoclase  gneiss,  quartzyte  and  crystalline 
limestone."' 

Toward  the  south  further  these  rocks  prevail  in  the  south- 
ern Adirondacks,  appearing  on  lake  Champlain.  They  contain 
in  Essex  county  the  titanic  iron  ore  which  has  been  mined,  for 
many  years.  They  are  here  associated,  in  some  manner  as  yet 
unascertained,  with  a  large  series  of  ([uartzose  gneisses,  crys- 
talline limestone  and  hematite  ores.  In  the  northern  slopes  of 
the  Adirondacks  these  latter  rocks  are  very  largely  developed, 
dipping  conspicuously  toward  the  north,  and  northwest.  A 
late  examination  by  the  writer  convinced  him  that  these 
gneisses  do  not  belong  in  the  true  Laurentian.  although  usually 
so  regarded.*  In  some  of  their  outcrops  they  seem  to  pass 
litliologically  into  the  quartzyte  so  largely  wrought  at  Potsdam, 
although  no  structural  evidence  was  found  to  support  sucli  a 
hypothesis.  In  many  places,  however,  in  eastern  New  York 
the  norytes  have  been  described  as  interbedded  with  such 
rocks,  and  in  the  Court landt  series,  on  the  Hudson  river  they 
embrace  nonconformable  masses  and  blocks  both  of  quartzyte 
and  of  limestone  known  to  be  of  primordial  age  by  the  dis- 
covery of  characteristic  trilobites.  The  series  extends  into 
N(.N\  Jersey,  carrying  valuable  iron  ores,  long  mined,  but  there 
the  mined  ores  are  uon-titanic  magnetites.  In  the  Courtlandt 
series,  according  to  the  descriptions  of  Prof.  Geo  H.  Williams  f 
all  transitions  occur  between  the  degree  of  presence  or  absence 
of  the  hornblendic,  the  augitic,  the  biotitic  and  the  chrysolitlc 
varieties.     The  olivine  bearing  portions  are  "destitute  of  any 

"iii'e,  however.  , I A.MES  llAi.i,.  Am.  .lourii.  Sci.,  (;i).  Xil,  2W,  who  e.xcliulos  tliotii   from 
tlio  Laiiroiitiiui. 

tAnierlciin  .loiiniiil  of  (•'ch'iR't!  (III).  XXX.  .Imii.  li^J-e,  p.  27. 


THE   NORIAN   OF   THE    NORTHWEST. 


XXIII 


considerable  quantity  of  feldspar,  and  belong  therefore  to  the 
family  of  peridotytes".  They  pass,  however,  by  an  increase 
in  the  amount  of  feldspar,  into  olivine  norytes,  olivine  gabbros, 
and  olivine  diorytes.  "The  constant  occurrence  of  such  trans- 
itional forms,  and  the  vi'ant  of  any  regularity  in  the  distribu- 
tion of  the  pure  tpyes.  make  it  impossible  to  regard  these  rocks 
as  anything  but  local  moditications,  or  special  facies  of  one  and 
the  same  mass.  However  great  their  mineralogical  variety 
may  be  they  together  form  but  a  single  geological  unit".  The 
age  of  this  invasion  of  the  fragmentals  at  Courtlandt  by  basic 
irruptives  is  certainly  i)aleozoic.  on  the  authority  of  Prof.  J. 
D.  Dana;*  and  Prof.  J.  F.  Kemp  has  stated  that  they  contain 
(at  Rosetown  near  Courtlandt)  masses  of  Cambrian  limestone. 
Dana,  indeed,  at  first  considered  them  metamorphosed  sedi- 
ments of  Lower  Silurian  age.  as  there  are  various  crystalline 
effects  ijroduced  by  them  on  the  concerned  elastics,  resulting 
in  schists  and  diorytes  and  soda-granites. 

The  same  rocks  have  been  found  by  C.  H.  Hitchcock  in  the 
White  mountains  of  New  Hampshire,  and  there  they  are  said 
to  lie  non-conformably  upon  an  older  metamorphosed  series, 
which  again  is  non-conformable  upon  the  true  Laurentian. 

A  formation  thus  characterized  and  so  easily  identified  by  its 
lithology  as  well  as  its  stratigraphic  relations  should  not  long- 
er be  without  a  recognized  name;  and  it  seems  appropriate  that 
it  should  bear  the  name  Norian.  whether  it  be  found  in  the  east 
or  in  the  northwest.  In  Minnesota,  and  in  the  north 
west  generally,  wherever  these  rocks  have  been  found 
they  are  associated  with  various  evidences  of  upheaval, 
metamorphism  and  eruption.  Therefore,  they  afford  not  only 
one,  but  numerous  interesting  features,  which  are  as  yet  but 
faintly  understood,  giving  rise  to  several  problems,  the  true 
solution  of  which  can  be  but  partially  foreshadowed,  or  are  en- 
tirely beyond  answer  with  our  present  Ivnowledge. 

THE   LOGAN    SILLS. 

Professor  Lawson's  second  paper,  the.  laccolitic  sills  of  the  Anl- 
inikie  rocks  of  lake  Superior,  has  a  close  relation  with  some  of  the 
problems  alluded  to  abo^'e,  and  his  investigation  throws  some 
light  on  them. 

It  had  been,  apparently,  the  source  of  some  perplexity  with 
Prof.  Irving,  that  the  gabbro,  wherever  he  saw  it  in  Wisconsin 
— and  the  same  is  true  of  it  in  Minnesota — is  non-bedded,  non- 


♦Amerlcan  .lourtiiU  of  Science,  (HI),  XX,  191. 1880. 


XXIV 


BULLETIN   NO.    VIII. 


amygdaloidal,  non-basaltic,  never  presenting  the  peculiar  stair- 
like ascents  up  the  hills  which  it  forms,  noted  by  Poster  and 
Whitney  as  characteristic  of  the  traps  of  Keweenaw  point.  Ir 
order  to  account  for  this,  and  for  some  other  peculiarities  of  its 
distribution  and  manner  of  outcrop,  he  imagined  that  perhaps 
the  gabbro  bosses  that  now  appear  at  the  surface  are  simply 
the  congealed  and  now  uncovered  reservoirs  of  basic  rock  ma- 
terial which  furnished  the  diabasic  overflows  of  the  Cuprifer- 
ous.    This  suggestion  he  repeats: 

"The  great  coarseness  of  grain,  the  perfection  uf  the  crystal- 
lization, the  abrupt  terminations  of  the  belts,  the  complete 
want  of  structure  and  the  presence  of  intersecting  areas  of 
crystalline  granitoid  rocks— all  suggest  the  possibility  that  we 
have  here  to  do  witli  masses  which  have  solidified  at  great 
depths.  They  certainly  cannot,  however,  be  regarded  as  in- 
trusives,  so  that  unless  we  regard  them  as  great  outflows,  we 
should  be  forced  to  look  upon  them  as  the  now  solidified  res- 
ervoirs from  which  the  ordinary  Keweenawan  flows  have 
come.'"* 

This  hypothesis,  though  rejected  by  Irving,  taken  in 
connection  with  the  facts  adduced  by  Dr.  Lawson  and  the 
further  fact  that  there  is  no  essential  petrographic  distinction 
between  the  gabbro  and  the  diabases  of  the  Keweenawan,  but 
that  attendant  physical  surroundings  woiild  account  for  all 
their  differences,  seems  to  explain  many  of  the  anomalies  which 
hitherto  have  hung  about  the  accepted  theory  of  origination  of 
the  gabbro.  In  northeastern  Minnesota  the  gabbro  belt,  along 
its  northern  border,  crowds  more  and  more  toward  the  north. 
It  sxTCcessively  traverses  the  southern  surface  boundary  of  the 
lower  Aniraikie.  or  Powabic  quartzyte,  with  which  it  is  con- 
spicuously interbedded,  then  rests  against  or  upon  large  areas 
of  the  Keewatin  (or  Kawishiwin)  greenstone;  and  finally  is  ab- 
solutely in  contact  with  granites  and  gneisses  of  the  Giant's 
range,  and  then  falls  rapidly  away  southward.  It  has  been 
supposed  that  as  a  surface  flow  the  gabbro  spread  back  north- 
ward and  thus  came  into  contact  with  these  older  rocks  in  the 
same  manner  as  the  Keweenawan  diabases  have  been  observed 
to  do;  but  it  may  be  that  this  movement  was  a  deep  seated  lac- 
colitic  motion  of  molten  rock,  entering  such  openings  in  the 
earth's  crust  as  the  incidents  of  fracture  and  upheaval  pre- 
sented.    Such  openings  were  liable  to  appear  not  only  in  the 


*Tlilr(l  aiuiiial  report  of  tlie  direetor  of  tlic  U,  S,  Oeologlcal  survey,  p.  12.5. 


the 


' 


THE   NORIAN    OF   THE    VOKTHWEST. 


XXV 


Animikie  Vjut  also  in  the  Keewatin.  the  lin(3  of  fracture  in  the 
crust  taking  such  a  direction  as  the  exigencies  of  pressure  or 
of  weakness  required.  Sucli  fracture  in  Minnesota  would 
appear  to  have  been  in  a  crescentic  line  with  is  concavity  toward 
the  southeast,  somewhat  more  sharply  curved  than  the  pres- 
ent shore  line  of  lake  Superior.  Starting  from  Dnluth  it  ran 
rapidly  north  or  northeastward.  Then  it  turned  more  east- 
wardly,  at  a  distance  of  about  seventy  miles  from  the  lake 
shore;  then  still  more  southeastward,  and  reached  the  shore 
again  at  Pigeon  point;  there  passing  under  the  present  water 
surface,  reappearing  apparently  on  the  northern  side  of  Isle 
Royale.  Such  fracture  line,  or  at  least  such  a  gabbro  belt, 
conforms  with  the  line  of  outflow-points  of  the  Keweenawan. 
and  with  the  laccolitic  appearances  of  the  trap  sills  in  the 
slates  of  the  Animikie  as  well  as  with  the  greatest  frequency  of 
diabase  dikes.  It  would  be  necessary  to  allow,  on  this  hypo- 
thesis, that  large  thicknesses  of  rocky  strata  luive  been  re- 
moved at  the  points  where  these  gabbro  reservoirs  gathered, 
in  order  to  explain  their  present  appearance  at  the  surface. 

As  to  the  date  at  which  this  laccolitic  disturbance  may  be 
supposed  to  have  taken  place,  it  is  plain  that,  if  it  gave  rise  to 
the  surface  flows  of  trap  wiiich  characterize  the  Keweenawan, 
it  must  have  been  substantially  coteuiporary  with  the  Kewee- 
nawan. The  Animikie  must  have  been  deposited  prior,  and  per 
haps  some  of  the  Keweenawan.  At  any  rate  the  escaping 
molten  magma  is  interbedded  with  and  constitutes  some  of  the 
Keweenawan  and  is  involved  with  some  clastic  beds.  Wherever 
the  Animikie  existed  it  was  liable  to  these  intrusions.  Where 
it  did  not  exist  the  laccolites  were  formed  in  older  rock. 
Where  the  batholitic  gabbros  are  directly  overlain,  as  at  Carl- 
ton peak  and  at  Beaver  bay,  by  Keweenawan  diabase  it  at  first 
appears  there  was  an  interval  of  surface  erosion  between  the  in- 
trusion of  the  massive  gabbro  and  the  extrusion  of  the  bedded 
diabase.  The  massive  structureless  knobs  of  gabbro  could  not 
thus  be  formed  at  the  surface  by  extrusion.  At  such  points  the 
Animikie  may  have  once  existed.  That  would  require  that  the 
grand  epoch  of  this  disturbance  should  have  its  commencement 
prior  to  the  bulk  of  the  Animikie;  should  continue  through  it 
and  should  have  its  close  in  the  later  portion  of  Keweenawan 
time.  If  the  Animikie  never  existed  at  such  points,  in  order  to 
produce  such  unconformities  there  must  have  been  eroded 
some  older  rocks  in  order  to  expose  the  gabbro  to  the  later  trap 
outflows.       Such   older   rocks   must  have   been   some   of   the 


XXVI 


BULLETIN    NO.    VIIL 


Archtoan,  and  in  that  case  some  traces  of  them  ought  to  be 
found  at  some  of  these  interesting  localities. 

The  non-existence  of  volcanic  debris  in  this  period  of  dis- 
turlnince,  when  such  deep-seated  movements  were  taking  place, 
allowing  the  transference  of  enormous  quantities  of  molten 
rock  from  place  to  place  within  the  crust,  and  the  extrusion  of 
other  enormous  quantities  at  the  surface,  and  that,  too,  at  a 
time  when  the  presence  of  the  sea  about  the  vents  is  attested 
by  the  occurrence  of  interbedded  clastic  rocks,  would  certainly 
be  an  unexpected  anomaly.  Dr.  Lawson  has  found  nothing 
that  indicates  the  existence  of  contemporary  volcanoes  in  the 
Animikie,  but  such  negative  evidence  is  not  quite  sufficient  to 
establish  such  an  important  principle.  In  this,  however,  while 
he  agrees  with  Prof.  Irving,  who  found  no  volcanic  ash  even 
In  the  Keweenawan,  he  is  at  variance  with  Foster  and  Whit- 
ney and  with  other  observers,  including  the  writer.  It  is  very 
probable  that  the  nature  of  some  of  the  soft  amygdaloids,  and 
some  of  the  stratiform  clastic  beds  of  the  Keeweenawan  have 
not  been  investigated  sufficiently.  It  is  very  certain  that  some 
beds  embraced  between  sheets  of  trap  on  the  north  shore  of 
lake  Superior,  consisting  of  wholly  non- consolidated  materials, 
have  the  appearance  of  being  of  the  nature  of  volcanic  ash, 
but  they  have  not  yet  been  examined  with  care. 

It  is  difficult  therefore,  with  this  interpretation  of  the 
gabbro  intrusion,  to  separate  structurally  and  chronologically 
the  later  diabase  extrusions  of  the  Cupriferous  from  the 
gabbro  itself,  just  as  it  is  to  distinguish  the  one  from  the  other 
by  any  essential  petrographical  characters.  If  this  hypothesis 
be  abandoned,  and  the  gabbro  belt  and  the  anorthosyte  rocks 
at  the  localities  described  by  Dr.  Lawson,  be  relegated  wholly 
to  Archscan  time,  as  suggested  by  him,  there  will  arise  such 
attendant  problems  that  it  may  be  found  impossible  to  satisfy 
all  the  facts.  For  instance,  if  the  gabbro  (anorthosyte)  dis- 
turbance was  in  ArchcJ3an  time,  and  the  diabase  outflows  and 
the  Animikie  laccolites,  were  produced  in  paleozoic  time,  and 
hence  subsequent  to  the  basal  Taconic  erosion -interval,  how 
does  it  happen  that  the  gabbro  (anorthosyte)  rocks  are  inter- 
bedded with  the  Pewabic  quartzyte  which  is  the  base  of  the 
paleozoic?  How  also  can  it  be  explained  that  where  the  gabbro 
(anorthosyte)  intrusions  approach  the  Animikie  slates,  form- 
ing large  bosses  such  as  Mt.  Josephine  and  Pigeon  point, 
there  also  the  diabase  dykes  and  laccolitic  sills  are  found  to  be 
largest  and   most   numerous?     It  may  reasonably  be    asked 


t 


THE   NORIAN    OF   THE   NORTHWEST. 


XXVII 


I 


also,  on  the  l^ypothesis  that  the  gabbro  (anorthosyte)  rock  is 
of  Archtuan  age,  how  can  its  close  petrographic  alliance  with 
the  diabases  be  accounted  for?  Probably  the  chief  obstacle 
to  the  separation  of  the  gabbro  (anorthosyte)  rock  from  the 
epoch  of  the  diabase  extrusions,  lies  in  the  close  areal  agree- 
ments which  they  manifest.  In  general,  the  area  of  one  is 
the  home  of  the  other.  Wherever  the  gabbro  occurs  there  is 
associated  traprock.  though  tlie  converse  is  obviously  not 
true.  The  trend  of  the  gabbro  lanorthosyte)  belt,  whether  in 
Wisconsin  or  Minnesota,  conforms  closely  with  the  strike  of  the 
diabase  belt.  On  the  south  side  of  lake  Superior  the  gabbi'o 
(anorthosyte)  Vjelt  is  to  the  south  of  the  northward  dipping  sur- 
face diabase  flows;  on  the  north  side  of  the  lake  it  is  on  the  north 
side  of  the  southward  dipping  diobase  tlows.  In  each  case  it 
allows  the  supposition  that,  on  the  degradation  of  the  rocks 
covering  the  batholite  while  tlie  reservoir  was  feeding  the 
surface  Hows,  the  uncovered  batholite  is  separated  geograph- 
ically from  the  diobase  tlows  by  a  surface  interval  which  maij 
be  occupied  in  whole  or  in  part  by  some  of  the  Animikie  beds. 
This  interval  is  frequently  hid  by  drift  accummulations  and 
has  rarely  been  studied  with  care.  However,  in  any  case, 
whether  there  be  any  intervening  Animikie  or  not.  if  the  in- 
trusion of  the  gabbro  took  place  within  Archtoan  rocks  at 
such  places  as  where  those  rocks  constituted  a  dry  land  sur- 
face, and  the  Animikie  had  never  been  deposited  over  them, 
the  diabase  extrusions  would  then  lie  directly  upon  the 
Archtcan,  and  on  being  uncovered  the  gabbro  would  be  found 
intcjrbedded  and  otherwise  in  contact  with  the  Arch;can  rocks. 
There  seems,  however,  to  be  no  ready  way  to  explain  the  im- 
mediate overlie  of  the  diabases  on  the  gabbro  (anorthosyte ) 
rocks,  as  described  at  Carlton  peak  and  eastward  from 
Beaver  bay,  and  as  mentioned  by  Prof.  Irving  in  Wisconsin, 
and  by  Poster  and  Whitney  on  Keweenaw  point,  except  by 
supposing  two  epochs  of  disturbance,  allowing  an  erosion  in- 
terval, or  one  long-continued  epoch  with  a  diversified  history 
including  surface  degradation  at  such  places.  Dr.  Lawson's 
illustrations  and  descriptions  in  this  bulletin  are  perhaj)s  the 
strongest  evidence  of  such  direct  unconformable  immediate 
superposition  of  the  diabase  upon  the  anorthosyte,  in  situ, 
but  it  is  yet  to  be  shown  that  those  contacts  a^'e  not  rather 
upon  detached  masses  of  the  anorthosyte  rock.  It  is  no  un- 
common thing  to  see  masses  of  the  anorthosyte  fifteen  or 
twenty  feet  in  diameter  wholly  embraced  in  the  diabase,  and 


XXVlll 


BULLETIN    NO.    VIIL 


in  some  instances  they  are  two  hundred  feet.  When  numerous 
such  masses  are  in  juxtaposition  it  is  plain  that  on  exposure 
and  glacial  degradation  they  would  present  at  the  surface  an 
appearance  of  continuous  rocic  in  situ.  The  lake  shore  line 
acting  on  such  a  mass  would  bring  out  the  "serrated"  coast 
noted  by  Dr.  Lawson,  and  the  individual  masses  would  appeal 
as  knobs  rising  above  the  more  rapidly  destructible  interven- 
ing diabase  matrix.  Such  a  deceptive  appearance  occurs  at 
Duluth.  It  is  the  most  obvious  inference,  based  on  casual 
examination,  that  at  that  place  the  melaphyrs  and  diabases  lie 
immediately  upon  the  gabbro;  but  between  the  actual  outcrops 
of  the  two  is  an  interval  rather  poorly  exposed  to  observation, 
in  which  have  been  seen  not  only  rounded  masses  of  gabbro, 
detached  from  the  main  hill  range,  but  also  some  rock  of 
obscure  characters  greatly  changed  from  its  original  and 
referable  to  some  of  the  basal  members  of  the  Animikie.* 
This  indefinite  rock  in  other  places  assumes  large  proportions 
and  appears  as  the  extensive  ''red  rock"  alternating  with  and 
cutting  the  gabbro  and  the  anorthosyto  rocks  as  at  Rica's 
point,  at  Beaver  bay,  and  at  Pigeon  jjoint.  It  has  been  ob- 
served at  many  places  to  pass  into  a  sedimentary  rock.  It  is 
frequently  i)lainly  a  conglomerate  originally.  This  belt  of 
changed  Animikie  appears  to  run  with  the  gabbro  from 
Duluth  to  Pigeon  point.  At,  the  latter  -p\-a>cc  Prof.  W.  S. 
Bayley  has  made  a  special  study  of  the  contact  phenomena  of 
the  gabbro  on  the  slates  and  quartzytes  and  his  conclusionst 
lately  piiblished  confirm  those  of  Richard  Owen  and  J.  G. 
Norwood,];  and  of  the  writer  published  in  many  jolaces.  to  the 
effect  that  the  red  rocks  of  the  Cupriferous,  including  the 
quartz  porphyries  and  red  syenites  specially,  are  modified 
conditions  of  sedimentary  rocks.  The  persistence  of  this 
modified  belt,  giving  rise  to  soda  granites  and  augite  syenites, 
or  to  "quartz  keratophyres,"  referable  to  pro-existing  (proba- 
bly) Animikie  strata,  precludes  the  non-existence  of  the  orig 
inal  Animikie  strata  throughout  any  wide  extent  between 
Duluth  and  Pigeon  point,  and  looks  toward  their  probable 
future  discovery,  perhaps  in  some  of  these  disguised  forms, 
in  the  vicinity  of  Beaver  bay,  and  hence  to  the  probably 
detached   condition   of    the   anorthosyte   rocks   on  which   the 

diabase  flows  are  seen  to  lie  at  that  place. 

^— .^^^^— ^^^— _—  I 

*Niiitli   uiiiiuiil   report,  p.  11.   under  "1   D;"    p.   V2,  iiiidor  "7:"  p.  1",   under  "42;' 
teutli  report,  i)p.  107-1(H>.  under  ••807"  and  "808." 

+  Am.  .Tour.  Scl.  XXXIX.  '^73,  1890. 

*  Report  of  ti  OeoloKlcal  Survey  of  Vv'isconsiii,  Iowa  and  Minnesota.  D.  D.  Owen. 


THE   NOUIAN    OF   THE    NORTHWEST. 


XXTX 


n  numerous 
n  exposure 
surface  an 
shore  line 
■ated"  coast 
luld  api)eai 
le  interven- 
3  occurs  at 
i  on  casual 
I i abases  lie 
al  outcrops 
bservation, 
of  gabbro, 
e    rock   of 
iginal    and 
Animilfie.* 
)roportions 
g  with  and 
at  Rica's 
s  been  ob- 
ock.     It  is 
is   belt   of 
)bro    from 
rof.    W.    S. 
nomena  of 
)nclusions+ 
and  J.  G. 
es,  to  the 
udi  ng    the 
modified 
!   of    this 
syenites, 
g  (proba- 
the  orig 
between 
probable 
ed  forms, 
probably 
iiich   the 

under  "412;' 


The  existence  of  this  extensive  "red  formation."  so  closely 
an  attendant  upon  the  gabbro  belt,  is  suggestive  of  still  another 
hypothesis.     Brielly  the  characters  of  this  rock  may  be  sum- 
marized.    It  is.  speaking  broadly,  a  rock  of  orthoclase,  horn- 
blende and  quartz.     The  predominating  orthoclase  is  sometimes 
so  coarsely  crystalline  as  to  be  macroscopically  evident,  but  is 
more  frequently   so  indefinite  as  to  show  no  crystalline  texture. 
Vjecoming  felsitic.     The  coarser  crystallization  is  found  in  the 
large  bosses  which  sometimes  make  mounlain-lilce  hills  and 
ranges.     The  finer  sorts  are  seen  as  fjuartz-porphyries.  felsytes. 
intersheeted  with  diabases  or  cutting  them,  and  as  veins  in  the 
gabbro.     The  feldspar  is  not  wholly  orthoclase.   but  is  some- 
times seen  to  be  a  striated   plagioclase,    which   Prof.    Irving 
considered  oligoclase.     Like  all  the  other  minerals  the  ortho- 
clase is  reddened  by  abundant  ferric  oxide.     This  has  been  con- 
sidered a  secondary  product  of  alteration,  but  it  is  more  likely 
to  have  been  originally  in  the  rock  from  which  the  "red  rock" 
was  derived.     The   ferro-magnesian  mineral  is  usually  horn- 
blende, but  Pi'of.  Irving  found  that  very  often  it  is  augitic.  He 
inferred  that  the  hornblende  is  wholly  derived  by  alteration 
from  original  augite.     It  is  very  certain  that  it  is  sometimes 
chloritized.     The  quartz  is  the  most  interesting  of  all  the  min- 
erals,  as   it  assumes  positions  and   forms   which  indicate   its 
origin.     In  some  cases  it  prevails  over  all  the  other  ingredients, 
making  a   quartzose   rock,   which   should  receive   ratlier   the 
name  gneiss.     Prof.  Bayley  gives  it  on  Pigeon  point  the  name 
"quartz-keratophyre."     Those  grains  of  (juartz  which  were   in 
the  original  rock  have  undergone  some  molecular  changes,  but 
usually  not  enough,  especially  in  the  finer  grained  portions,  to 
destroy  their  clastic  characters.     The  "club-shaped"  quartzes 
seem  in  the  red  augito-syenites  associated  with  the  gabbro  at 
Duluth,  were  pronounced  to  be  secondary  quartz  by  Prof.  Irving.* 
and  that  which  had  been  so  completely  fused  as  to  crystallize  in- 
dependently amongst  the  orthoclases,  was  considered  by  him  as 
the  original  quartz.     The   reverse  may  be   true,   the   original 
fragm.ental  quartz  grains  having  been  drawn  out  into  the  forms 
which  are  illustrated,  and  some  of  them  rearranged  as  to  out- 
ward form,  and  all  of  them  as  to  crystallographic  directions. 
When  complete  fusion  was  superimposed  on  the  sedimentary 
mass,  the  quartz  grains  were  blended  in  one,  and  on  cooling 
were    forced    to  accommodate    themselves    to    the    adjoining 
orthoclases.     In  the  felsytes   the  quartzes   are  sometimes    in 


D.  D.  Owen. 


*See  parlloiiliirly  Moii.V,  V.  S.  Oeol  Sur.,  PI.  XlVaiid  the  acconipanyiiig  Uescripiloii. 


XXX 


BULLETIN    NO.    VJII. 


semiroundod  grains  and  sometimes  are  doubly-terminated 
crystals.  These  rocks  show  almost  every  conceivable  manner 
of  association  with  the  gabbro.  They  cut  it  in  dikes,  both  per- 
pendicular and  inclined,  and  they  swell  out  in  large  patches, 
surrounded  by  the  gabbro.  They  underlie  and  overlie  large 
areas  of  gabbro,  and  both  are  cut  by  the  later  dikes  of  diabase. 
They  are  cut  olT  by  perpendicular  bosses  of  the  gabbro,  some- 
times having  a  perpendicular  or  inclined  line  of  close  contact 
without  blending,  and  sometimes  the  elements  of  the  two  rocks 
are  united  as  by  a  mixture  in  one  common  magma,  making 
them,  apparently,  the  so-called  orthoclase  gabbro,  as  seen  at 
Duluth.  Considering  the  diabases  of  the  early  portion  of  the 
Keweenawan  as  derivatives  from  the  gabbro  batholites,  these 
rocks  when  cutting  and  alternating  with  the  diabases,  may  in 
like  manner  be  considered  as  cotemporary  derivatives  of  large 
batholites  or  other  masses  of  red  rock  ready  to  enter  any  open- 
ings in  the  surrounding  crust  where  they  could  tind  relief  from 
thecrustal  pressure.  As  pebbles,  they  are  more  durable  than 
those  of  gabbro,  and  they  constitute  large  beds  of  conglomer- 
ate in  the  base  of  the  copper-bearing  rocks.  They  are  found 
as  transported  blocks  or  boulders  in  association  with  trans- 
ported blocks  and  boulders  of  the  gabbro  inclosed  in  the  later 
traps,  as  already  fully  noted  at  Beaver  bay,  and  about  Encamp- 
ment island. 

Chemical  Analyses  of  the  Red  Rock. — For  purposes  of  further 
comparison  the  following  table  is  compiled  of  such  analyses  as 
are  at  hand  of  this  "red  rock"  formation: 

I.  From  the  west  bluff  at  the  entrance  to  Beaver  bay  harbor. 
Analysis  by  J.  A.  Dodge.  Building  stones  of  Minnesota.  Vol. 
1,  of  the  final  report,  p.  198.     Survey  No.  124. 

II.  Rice  point,  red  granite;  by  Prof.  J.  A.  Dodge.  Tenth 
annual  report;  p.  204,  Sur.  No.  1  B. 

III.  Another  analysis  of  the  same  as  II,  Prof.  J.  A.  Dodge. 
Thirteenth  annual  report,  p.  100  (No.  148). 

IV.  Finely  crystalline  brown  syenite;  Duluth,  J.  A.  Dodge. 
Geol.  Sur.  No.  7.     Thirteenth  report,  p.  100  (No.  149). 

V.  Fine  grained,  reddish  brown  rock;  Duluth,  J.  A.  Dodge. 
No.  19  of  the  Geo.  Sur.  series.  Thirteenth  annual  report,  p. 
100  (No.  150). 

VI.  "Streamed,"'  light  red,  with  translucent  laminations 
and  specks.  London  (near  Duluth).  J.  A.  Dodge,  Thir- 
teenth annual  report  p.  100  (No.  152).     Geol.  Sur.  No.  68. 


I 


THE   NOUIAN   OF   THE   NOUTHVVEST. 


XXXI 


VII.  Brick  red,  rather  fragile,  apparently  j?ritty  and  sub- 
crystalline.  J.  A.  Dodf?e.  About  two  miles  east  of  the  mouth 
of  Passabika  (Lester)  river,  near  Duluth.  Thirteenth  annual 
report,  p.  100  (No.  IJS). 

VIII.  Purplish  rod  ufranite  from  the  west  bluff  at  the 
entrance  to  Beaver  bay.  J.  A.  Dodge.  Another  analysis  of 
the  same  rock  as  No.  I.  Geo.  Sur.  No.  124.  Thirteenth  annual 
report,  p.  100  (No.  155). 

IX.  Grayish-red   folsitic   rock,  in 
metamorphic  cpiartzyte  or  quartzose 
interlaminations  of    ••streams"    of    siliceous    matter. 
Dodge.     Thirteenth  annual    report,   p.  100  (No.    l^S). 


the  field  taken  to  bo  a 
shale,  rather  slaty,  with 
J.    A. 

Geol. 


Sur.  No.  127.  The  analysis  shows  this  rock  to  bo  nearly  iden- 
tical with  VIII,  with  which  it  probably  has  a  connection.  They 
outcrop  near  adjacent,  at  the  west  side  of  Beaver  bay. 

X.  Red  granite  from  the  third  island  below  Beaver  bay.  J. 
A.  Dodge.  Thirteenth  annual  report,  p.  100  (No.  157).  Geol. 
Sur.  No.  184. 

XI.  Rock  of  the  bulk  of  the  Great  Palisades,  a  red  quartz 
porphyry.  J.  A.  Dodge.  Thirteenth  annual  report,  p.  100 
(No.  158).     Geol.  Sur.  No.  139. 

XII.  Red.  laminated,  or  •streamed,"  at  the  base  of  the 
Great  Palisades.  J.  A.  Dodge.  Thirteenth  annual  report,  p. 
100  (No.  159).     Geol.  Sur.  No.  140. 

XIII.  The  red  rock  at  Grand  Marais;  furnishes  the  pel^bles 
of  the  beach.  J.  A.  Dodge.  Thirteenth  annual  report,  p.  100 
(No.  162).     Geol.  Sur.  No.  203. 

XIV.  Red  rock  from  the  first  island  northwest  of  Belle 
Rose  island,  south  of  Pigeon  point.  J.  A.  Dodge.  Thirteenth 
annual  report,  p.  100  (No.  164).  Geol.  Sur.  No.  285.  This 
seems  to  be  the  augite  syenite  examined  by  Prof.  Irving,  from 
Brick  island. 

XV.  Analysis  of  the  powder  of  seven  specimens  of  the 
granular  varieties  from  Pigeon  point;  W.  P.  Hillebrand.  Re- 
ported by  Prof.  W.  S.  Bay  ley.     Am.  Jour.  Sci.  (3).  xxxvii,  59. 

XVI.  Powder  of  three  of  the  quartz  porphyries  from  Pigeon 
point;  W.  F.  Hillebrand.  Reported  by  Prof.  W.  S.  Bayley. 
(A.  J.  Sci.) 

XVII.  For  purposes  of  comparison  this  analysis  was  made 
of  one  of  the  associated  clastic  rocks.  It  is  from  a  pinkish 
quartzyte  at  the  head  of  Wauswaugoning  bay,  near  Pigeon 
point,  a  rock  which  is  found  in  immediate  contact  with  the  red 
rock  and  with  the  gabbro  on  Pigeon  point.  J.  A,  Dodge;  thir- 
teenth annual  report,  p.  100,  (No.  163.)    Geol.  Sur.  Series  262^ 


XXXll 


UULLETFN    NO.    VIII. 


XVIII.  This  analy.sis  was  also  made  for  comparison.  It  is 
of  a  rod  sandstone,  or  quartzyto,  ftntv^i'ainod.  from  tlio  north 
sido  of  Siskiwit  point.  Isle  Royal,  formerly  quarried  for  build- 
ing stone.  J.  A.  Dodge;  thirteenth  rei^orl,  p.  100  (No.  10')). 
Cieol,  Sur.  Series  555. 

An  inspection  of  this  table  of  analyses  will  convince  anyone 
not  only  that  the  red  rock  series  is  a  geologic  unit,  extending 
from  Dululh  to  Pigeon  point,  thus  agroeeing  with  the  result 
derived  from  field  evidence,  many  times  reported,  and  with 
microscoiiical  examination,  but  that  it  is  referable  to  a  change 
of  sedimentary  rocks  immediately  adjoining  th(5  gabbro  bosses, 
and  in  that  also  agreeing  with  the  results  of  extended  field 
studies,  the  most  thorough  of  wliich  are  those  of  Prof.  W.  S. 
Bayley  on  Pigeon  point. 

It  is  therefore  a  legitimate  inference  that  where  this  red  rock 
is  most  abundant,  there  the  gabbro  acted  most  powerfully  and 
profoundly  on  the  strata  concerned.  The  Animikie,  therefore, 
must  have  been  fused  through  that  whole  bolt  where  this  red 
rock  prevails — that  is,  in  all  that  region  north  and  northeast- 
ward from  Grand  Marais,  and  also  westward  toward  Beaver 
Bay,  and  finally  to  the  vicinity  of  Duluth,  through  which  the 
Animikie  rocks  ought  to  be  found,  unless  some  such  unusual 
event  supervened  to  cause  their  disappearance  by  converting 
them  into  this  pu/zling  red  ro"k. 

It  is  not  necessary  here  to  dwell  on  the  importance  of  such  a 
result.  The  writer,  as  he  pursues  this  review,  becomes  im- 
pressed with  the  probable  correctnesss  of  this  last  hypothesis, 
and  is  constrained  to  adopt  it  as  a  working  hypothesis  for 
future  research. 

Lithologlsts  have  looked  at  the  augite  syenytes  from  the  side 
of  (U'iginal  fusion  and  eruption,  and  all  their  microscopic 
characters  have  been  interpreted  and  defined  in  terms  of 
modern  petrography,  which  hardly  yet  may  it  be  said  to  have 
recognized  any  other  source  for  crystalline  rocks.  Hence  in 
reading  the  descriptions  of  these  rocks  (as  those  by  Irving) 
many  ideas  of  genesis  and  alteration  must  be  reversed.  It  is 
also  difficult  to  distinguish  fact  from  hypothetical  presump- 
tion. Facts  and  presumption  are  expressed  with  equal  posi- 
tiveness— nay.  sometimes  presumption  is  made  more  positive 
than  fact  by  the  addition  of  adverbs  and  phrases  that  are 
intended  to  highten  the  author's  assurance  and  exclude  the 
reader's  possible  doubt,  as  if  there  were  no  possibility  of  error 
in  the  initial  data. 


TIIK  NdUIAN  OF  THK  NORTFiWKST. 


X  X  XIII 


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\X.\IV 


ML'I.I-KTIN    NO.    VIII. 


ir  lh«!st)  rod  rocks  ai'o  rot'urabU'  to  tlio  cuiis*!  Iiort!  suppo.swl. 
vi/:  a  Hocondary  prodiu^t  l)y  fusion  of  sodiriK^ntary  rocks,  it 
will  bo  nt'ccs.sary  to  inako  profound  cluui^'os  in  Iho  inotliods 
and  masoning' of  some  iiiirrosco])ical  students.  Thoy  must  begin 
thoii*  study  from  tlio  opposite  etui  of  the  i)roc(!ss.  The  inici'o 
s(tope  must  reverse  its  line  of  i)ursuit.  and  s<!arch  foi'  cause 
where  it  lias  been  accustomed  to  find  (effect.  It  must  accept  as 
orlf/inal  featureK  some  things  that  it  has  called  secondary. 
"Secondary"  silica,  "corrosion."*  "alteration"  and  "dynamic 
etfects."  described  in  these  and  probably  other  rocks  as  results 
of  change  from  an  earlier  completely  crystalline  condition, 
must  be  considered  remnants  of  a  former  stratified  sedlm«m- 
tary  structure.  The  student  must  pi'ocotid  from  thes«j  data, 
and  other  more  evident  data  indicating  the  original  state 
of  these  rocks. .to  build  up  a  rational  theory  of  their  doveloj)- 
ment  into  crystalline  masses.  This  is  a  groat  tield  yet  unex- 
plored and  oxu\  of  pro])hetic  })otentialities  for  the  Archii^an 
rocks. 

There  are  various  other  and  important  questions  that  spring 
up  in  connection  w'th  this  view  of  th(!  geognosy  of  the  north- 
eastern portion  of  the  state,  some  of  which  may  be  alluded  to. 

1.  What  was  the  source  of  the  material  which  formed  the 
gabbro  batholites — was  it  deep  seated  or  was  it  within  the  eai-th's 
super- crust  V 

'2.  What  portion  of  this  red  rock  is  contempoi-ary  with  the 
gabbro  itself  in  origin,  and  what  pait  is  simply  the  proti'uding 
knobs  of  older  Laurentian  with  which  the  l)asic  rock  happened 
to  come  in  contact  when  it  was  seeking  a  place  of  ec|uilibrium 
within  t!ie  semi -plastic  crust. 

;5.  Has  the  (Jiant's  range  granite  any  relation  of  cause  and 
effect  with  the  gabbroV  In  other  words,  is  the  granite  of  tlie 
Giant's  range  a  keratophyre.  or  is  it  to  be  associated  with  the 
Laurentian  or  due  to  re-crystalline  sediments  of  the  Keewatiny 

4.  What  was  the  effect  of  the  gabbro  upon  the  older  sedimen- 
taries,  particularly  upon  the  fragmentals  of  volcanic  origin 
found  in  the  Keewatin? 

These  questions,  if  solved,  would  probably  give  rise  to  others, 
and  so  on  in  a  limitless  series.  The  student  never  finishes  his 
pursuit  by  the  capture  of  ultimate  results.  He  is  simply  lured 
on  by  present  quest — may  be  he  may  add  to  the  sum  of  human 
knowledge.  These,  however,  are  some  of  the  problems  which 
will  concern  some  future  students  of  these  terranes. 


y. 
y 


y. 


I 


THE  ANORTHOSVTES  OF  THE  MINNESOTA  SHOKE 
OF  LAKE  SUPEKIOl!. 


:1 


Br 

1 

■/.  Z. 

E-  7 

-  - 

1 

J: 

>  - 

.ii; 

1 

INTRODUCTION. 


The  Minnesota  coast  of  lake  Superior  between  Duluth  and 
Grand  Portage  was  mapped  and  described  by  Irving,  *as  exclu- 
sively occupied  by  formations  of  the  Keweenian  (Keweenawan) 
series,  and  his  discussion  of  the  geology  of  the  coast  is  cer- 
tainly the  most  sjctematic  which  has  up  to  the  present  ap- 
peared. State  Geologist  Winchell  had  earlier  made  careful 
examinations  along  the  same  line  of  outcrop,  and  had  placed  on 
record  a  host  of  detailed  observations  fon  the  rocks  of  the  coast, 
without,  at  that  time  expressing  any  final  conclusions  as  to 
their  stratigraphy  and  general  relations.  Quite  recently,  how- 
ever, we  have  an  expression  of  Prof.  Winchell's  views  of  the 
geology  of  the  coast  of  Minnesota  in  the  form  of  a  map  issued 
by  the  State  Survey.  jProm  this  map  we  gather  that  Prof. 
Winchell  is  in  accord  with  Irving  as  to  the  Keweenian  age  of 
that  portion  of  the  coast  which  lies  between  Duluth  and  Gi'and 
Marais  excepting  two  small  areas  at  Beaver  bay  and  the  vicin- 
ity of  the  Palisades;  but  differs  from  him  in  classing  as  "Norian" 
the  rocks  at  Duluth  and  the  most  of  those  between  Grand 
Marais  and  Grand  Portage,  together  with  those  of  the  small 
areas  above  mentioned  at  Beaver  bay  and  the  Palisades.  This 
correlation  of  the  Duluth  gabbro  and  the  black  gabbros  east 
of  Grand  Marais,  together  with  the  red  granites,  quartzi)or- 
phyries  and  red  felsytes  of  the  coast,  as  a  set  of  formations 
equivalent  to  the  Norian  is  a  suggestion  which  the  writer  is 
not  prepared  to  follow.  As  it  is,  however,  simply  an  expres- 
sion of  opinion,  not  yet  supported  by  the  publication  of  the 
facts  and  arguments  which  influenced  Prof.  Winchell  to  its 
adoption,  it  calls  for  no  criticism. 

But  it  is  the  accordance  of  Winchell  and  Irving  rather  than 
their  differences  of  opinion  to  which  the  writer  desires  to  direct 
attention,  in  order  to  make  clear  the  views  which  obtain  in  the 
literature  of   the   subject,    as  to  the  absence  on  this  coast  of 

•(^opper-heai'lriR  Koeks  of  Lake  Superior.— MunoKi'iiph  V.  U.  S.  G.  S.  Cup.  VII. 
tGeol.  ami  Nat.  Hist.  Survey  of  MiniiCHUla  Ulli  and  lOtb  Annual  llepurts. 
WJeoloKlcal  map  of  the  Iron  Regions  of  Minnesota  by  N.  H.  Wincliell,  and  H.  V 
Winoheli,  1890.     Auuompauyiug  "Iron  Ores  of  Minnesota." 


BULLETIN   NO.    VIU. 


rocks  of  any  geological  horizon  other  than  the  Kewecaian,  or 
than  those  classed  by  Winchell  as  Norian.||  The  preposition 
which  the  writer  lays  down  in  this  paper  is  that  he  has  dis- 
covered on  the  Minnesota  coast  a  geological  formation  of 
strongly  marked  individuality,  which  is  distinctly  older  than 
the  Keweonian,  and  is  separated  from  it  by  a  period  of  pro- 
found erosion.  This  formation  is  in  no  way  connected  with 
the  rocks  classed  by  Winchell  as  Norian.||  The  rock  of  which 
this  formation  is  composed  has  been  described  by  both  Winch- 
ell* and  Ix'ving.f  and  was  also  observed  by  Norwood  I  Nor- 
wood and  Winchell  first  called  the  rock  "feldspar  rock"  and 
the  latter  named  the  feldspar  of  which  it  is  composed  labra- 
dorite,  subsequently  referring  to  the  rock  as  "labradorite  rock." 
Irving  after  a  seemingly  thorough  examination§  of  the  feld- 
spar determined  it  to  be  anorthite  and  referred  to  the  forma- 
tion as  "anorthite  rock."  None  of  these  observers  seems  to 
have  apj^reciated  the  significance  of  the  field  evidence  as  to 
the  true  relations  of  the  formation  to  the  Keweenian,  and  in- 
deed no  discriminative  effort  of  any  kind  has  been  made  to 
separate  it  from  the  prevailing  series  of  the  coast. 

It  is  the  purpose  of  this  paper  to  set  forth  those  relations, 
and  to  establish,  by  clear  and  unequivocal  evidence,  the  ex- 
istence on  this  coast  of  a  new  geological  horizon  of  much 
interest. 

The  fact  that  the  rock  has  been  \ari()usly  named  by  the  only 
investigators  who  have  published  descriptions  of  it  renders  it 
necessary  that  the  evidence  of  its  true  petrographical  char- 
acter should  be  first  passed  in  review,  particularly  as  the  rock 
is  in  itself  an  interesting  type  whose  importance  is  not  yet 
fully  recognized  in  petrographical  literature.  Following  the 
petrographical  notes,  will  be  given  a  brief  account  of  the  dis- 
tribution of  the  formation  along  the  coast,  and  this  will  be  fol- 
lowed by  a  discussion  of  its  relations  to  the  Keweenian,  and  of 
its  probable  correlation  as  a  pre  Keweenian  horizon. 


PETKOQKAPHICAL  CHARACTERS. 

Irving's  dcscripflon: — Irving's  summary  statement  of  the  pet- 
rographical character  of  this  formation  is  as  follows:  "At 
several  points  on  the  north  or  Minnesota  shore  of  lake  Super- 

♦Nlnth  Annual  Report,    I'p.  20,  HO.    Tenth  Annual  Report,    p.  ;«). 
+c;()l)|)er-»eariii(;  rocks  of  Lake  Superior,     p.  riD-til ;  •j:!8-440. 

JUoportof  II  Oeolottical  .Survey  of  Wlsi-onsiu.   lowii  and  Minnesotii,  by  I).  D.  tUven. 
1852.    pp.  ;tOO,  301,  380. 
nXliese  are  .some  of  tlie  rocks  ol«s.sed  by  Wiueliell  as  Nortan.    LN.  H.  W.l 
HLoc.  ult. 


THE   AN0RTH0SYTF:S    of   MINNESOTA. 


3 


jraian,  or 
•c  position 
e  has  dis- 
nation  of 
)lder  than 
)d  of  pro- 
cted  with 
:  of  which 
th  Winch- 
)d  t    Nor- 
rock"  and 
ied  labra- 
i-ite  rock." 
the  feld- 
the  forma- 
s  seems  to 
ence  as  to 
an,  and  in- 
n  made  to 

3  relations, 
36,  the  ex- 
1  of  much 

jy  the  only 
.  renders  it 
hical  char- 
is  the  rock 
is  not  yet 
lowing  the 
of  the  dis- 
will  be  fol- 
lian,  and  of 


of  the  pet- 
ows:  "At 
ake  Super- 

by  1).  D.  Owen. 


ior.  between  the  mouth  of  Splitrock  river  and  the  Great  Pali- 
sades, and  again  in  the  high  point  near  the  mouth  of  Temper- 
ance river,  known  as  Carlton  peak,  are  to  be  seen  exposures 
of  a  very  coarse,  light  gray  to  colorless  or  white  rock,  occas- 
ionally with  a  faint  greenish  tinge.  This  is  seen  in  thin  sec- 
tions' to  be  composed  exclusively  or  nearly  so  of  anorthite 
feldspar.  Often  there  is  no  other  mineral  present  except  in 
exceedingly  minute  inclusions  and  these  are  very  sparse.  In 
one  section  a  few  gi'ains  of  altered  olivine  were  noticed  within 
the  anorthite,  and  in  two  or  three  a  little  augite  between  the 
feldspar  grains.  The  feldspar  appears  in  every  case  to  be 
anorthite.  In  no  section  did  it  show  the  peculiar  arrangement 
of  needle  like  inclusions  met  in  Euroiiean  gabbros,  and  so  com 
mon  in  the  coarse  gabbros  of  lake  Superior,  to  which  this  rock 
is  very  nearly  related." 

Supplementary  description: — This  description  requires  to  be 
both  supplemented  and  modified.  One  of  the  most  remarkable 
features  of  the  rock  is  its  extreme  freshness.  The  constituent 
feldspar  of  the  rock  is  never  found  in  a  decomposed  condition, 
but  presents  on  the  contrary  uniformly,  in  all  parts  of  the  for- 
mation, perfectly  glassy  and  brilliant  appearance. 

Another  remarkable  characteristic  of  the  rock  as  a  formation 
is  its  massiveness  and  lack  of  structural  planes  of  any  kind  ex- 
cept under  some  abnormal  circumstances  which  will  be  more 
fully  mentioned  in  the  sequel.  Even  jointage  may  be  said  to  be 
entirely  absent.  Occasionally  the  formation  is  traversed  by 
one  or  moi-e  fissures  locally,  but  these  follow  no  law  of  direc- 
tion and  cannot  be  regarded  as  true  jointage.  There  are  few 
of  the  massive  roclcs  of  the  lake  Superior  region  so  free  from 
structural  planes.  There  is  no  flow  or  gneissic  structure,  and 
the  rock  seems  nowhere  to  have  been  subjected  to  forces  which 
would  tend  to  deform  the  mass,  render  it  schistose  or  in  any 
way  induce  the  development  of  secondary  structures  of  a  me- 
chanical nature. 

The  color  of  the  weathered  surface  of  the  rock  is  prevail- 
ingly white  or  yellowish  white.  The  surfaces  are  usually 
smooth  in  general,  but  minutely  rough,  showing  often  the  solv- 
ent action  of  meteoric  waters  in  the  same  way  that  marbles  and 
limestones  do.  though  in  a  less  marked  degree.  The  weath- 
ered layer  is  very  thin,  and  the  fresh  glassy  crystals  are  ap- 
parent through  it.  The  color  of  the  rock  on  fractured  surfaces 
is  usually  as  Irving  described  it,  but  there  are  also  darker, 
greenish-gray  facies. 


BULLETIN    NO.    VIIL 


The  texture  is  prevailingly  coarse,  the  cleavage  faces  on  the 
conslitutent  feldspar  being  frequently  half  an  ipch  in  diameter 
(occasionally  an  inch)  and  ranging  from  that  to  one  quarter  of  an 
inch.  In  some  localities  it  talces  ou  a  finer  texture,  and  would 
then  be  designated  a  medium  grained  rock.  The  structure  of 
the  rock  is  eminently  allotriomorphic- granular  throughout  the 
entire  formation.  No  porphyritic  crystals  are  anywhere  ob 
servable,  and  under  the  microscope  no  suggeston  of  idiomorph- 
ic  forms  could  be  detected. 

The  Const'duenf  Feldspar — The  rock  is,  as  former  observers  have 
noted,  composed  almost  wholly  of  a  plagioclase  feldspar.  In  the 
majority  of  thin  sections  no  other  mineral,  except  in  the  form 
of  minute  interpositions,  is  present,  and  in  the  few  sections  in 
which  a  ferro-magnesian  silicate  may  be  observed,  it  plays  only 
an  accessory  r(')le.  This  plagioclase  is  not,  however,  as  Irving  af- 
firmed, always  anorthite.  Neither  is  it,  as  Winchell  believed, 
always  labradorite.  In  an  attempt  to  reconcile  the  conflicting 
statements  of  these  two  geologists,  the  writer  has  discovered 
that  some  portions  of  the  formation  accord  with  Irving's  deter- 
mination and  other  portions  with  Winchell's;  that  both  were 
probably  right  as  regards  the  material  which  they  subjected  to 
examination,  but  that  both  erred  in  generalizing  as  to  the  uni- 
form character  of  the  feldspar  throughout  the  formation.  The 
determination  of  the  feldspar  by  optical  and  chemical  exami- 
nation is  rendered  comparatively  easy  by  its  perfectly  fresh 
condition,  its  pronounced  cleavage,  the  constant  presence  of 
strongly  marked,  polysynthetic  twinning,  and  by  the  ease  with 
which  material  free  from  interpositions  can  be  selected  for  , 
analysis.  These  interpositions  are  not  uniformly  distributed, 
and  owing  to  the  glassy  transparent  character  of  the  crystals, 
it  is  a  simple  matter  to  separate  among  small  fragraments, 
pieces  which  are  perfectly  clear  and  vitreous  from  those  which 
are  more  or  less  charged  with  inclusions.  With  such  favorable 
material  a  portion  of  the  rock  formation  was  definitely  de- 
termined to  be  composed  of  labradorite.  ' 

Optical  Measurements: — For  optical  observations  a  number  of 
thin  sections  were  prepared  strictly  parallel  to  the  basal  pina- 
coid,  and  to  the  brachy  pinacoid,  and  numbers  of  readings  were 
taken  for  the  extinction  of  adjacent  twin  lamellae.  The  follow- 
ing results  were  thus  obtained,  the  figures  given  being  the 
mean  extinction  for  the  two  sets  of  slightly  discordant  readings 
on  either  side  of  the  cross  hair  of  the  microscope.    The  cleav- 


% 


THE  ANORTHOSYTES  OF  MINNESOTA. 


es  on  the 
diameter 
,rter  of  an 
,nd  would 
iicture  of 
^hoilt  the 
vhere  ob 
iomorph- 

•vers  have 
ir.   In  the 
the  form 
actions  in 
)lays  only 
Irving  af  • 
believed, 
jonflicting 
liscovered 
]g's  deter- 
both  were 
bjected  to 
o  the  uni- 
tion.    The 
;al  exami- 
ctly  fresh 
esence  of 
ease  with 
ected   for  , 
stributed, 

crystals, 
graments, 
ose  which 

favorable 
litely  de- 

lumber  of 
asal  pina- 
ings  were 
he  follow - 
jeing  the 
readings 
'he  cleav- 


age fragments  selected  were  taken  from  material  collected  on 
the  shore  near  Encampment  island  and  from  Carlton  peak. 
Extinction  Angle  Extinction  Angle 


i 


4 


onO.  P.  (001) 


on  au  P  ^  (010) 


21° 

17 

21° 

27 

21° 

27' 

22° 

21' 

23° 

33' 

25^ 

00' 

8°  4r 

9°  00'  . 
9°  02' 
9°  07' 
9°  08' 
9°  09' 
9°  12' 
9°  18' 
9^  22' 
9°  55' 

10°  49' 
10^  57' 
11°  00' 

These  results  locate  the  feldspar  in  the  plagioclase  series 
somewhere  between  Ab3  An^  and  Abj  Auj,  and  it  may  there- 
fore without  hesitation  be  designated  a  labradorite. 

Coujirhiatory  tests  : — In  order  to  contii'm  this  conclusion  the 
material  from  near  Encampment  island  was  subjected  to 
further  tests.  A  calculation  based  upon  the  extinction  angles 
above  recorded,  showed  that  the  composition  of  the  feldspar 
was  about  Ab,  An.j  and  the  chemical  composition  was  predicted 
for  it  as  it  is  given  in  column  I  of  the  table  of  analyses.  Care- 
fully selected  material  was  then  analysed  (by  W.  C.  Blasdale. 
Fellow  in  Chemistry  in  the  University  of  California)  and  the 
results  obtained  are  given  in  the  adjacent  column,  II.  This 
analysis  again  establishes  the  labradorite  character  of  the 
feldspar.  Its  specific  gi'avity,  as  determined  on  glass- clear 
fragments  with  the  aid  of  Klein's  solution,  is  '2.701'  which  is  in 
entire  accordance  with  its  optical  and  chemical  character.  The 
powdered  mineral,  moreover,  is  insoluble  after  prolonged 
boiling  in  hydrochloric  acid,  the  powder  being  in  appar- 
ently the  same  condition  at  the  close  of  the  operation  as  at  the 
beginning.  There  is  thus  no  doubt  whatever  as  to  the  mineral 
being  labradorite. 

Bock  composed  of  Labradorite  : — No  other  feldspar  than  the 
one  examined  can  be  detected  in  the  rock.  Indeed  the  material 
selected  for  analysis  was  not  from  a  single  crystal  but  from 
crystals  in  various  parts  of  the  hand  specimen.  It  is  there- 
fore believed  that  the  whole  rock  at  this  place  is  composed  of 
labradorite  feldspar  and  that  no  anorthite  is  present.     This 


€ 


BULLETIN    NO.    VIH. 


view  is  sustained  by  the  results  obtained  for  the  specific 
gravity  of  the  rock  as  a  whole.  Three  fragments  of  the  rock 
taken  at  random,  and  ranging  in  weight  from  three  grammes 
to  sixteen  grammes  were  used.  The  three  results  were  2.702. 
2.704  and  2.706.  The  mean  of  these,  viz  :  2.704.  varies  so  little 
from  that  of  pure  labradorite,  that,  in  the  total  absence  of  any 
lighter  non-feldspathic  minerals,  it  may  be  safely  inferred  that 
the  rock  is  composed  practically  of  one  feldspar,  viz:  labra- 
dorite. A  bulk  analysis  of  the  rock,  made  upon  fragments 
taken  at  random,  by  Mr.  Chas.  Palache,  Fellow  in  Mineralogy 
in  the  University  of  California,  is  given  in  column  III  of  the 
table,  and  also  demonstrates  the  fact  that  the  rock  as  a  whole 
has  essentially  the  composition  of  labradorite. 

Roc/i  compofied  of  Anorthite: — ^A  specimen  from  another  local - 
y,  viz:  from  the  cave  to  the  east  of  Split-rock  point,  was 
broken  into  small  fragments  and  pure  transparent  vitreous 
fek^spw  was  again  selected  for  analysis.  This  was  submitted 
!o  i'l  >'.  J.  A.  Dodge  of  the  University  of  Minnesota,  and  the 
resuUs  of  iiis  analysis  are  given  in  column  IV  of  the  table. 
This  analysis  shows  that  the  feldspar  at  this  locality,  though 
rather  rich  in  silica,  is  essentially  anorthite,  the  alkalies  being 
very  subordinate  in  amount,  and  this  confirms  the  correct- 
ness of  Irving's  optical  determinations  of  the  feldspar  of  this 
rock  from  certain  localities  on  this  coast.  Irving's  analysis* 
of  a  specimen  from  a  point  two  miles  below  Beaver  Bay  is 
given  in  column  V,  but  the  relative  proportions  of  lime  and 
soda,  as  well  as  the  specific  gravity,  suggest  that  the  feldspar 
examined  is  bytownice  rather  than  anorthite. 

Chemical  Armlyses: — The  following  table  gives  the  result  of 
a  few  chemical  analyses  which  have  been  made  of  these  rocks 
and  of  their  constituent  feldspars, f  together  with  two  analyses 
quoted  for  comparison: 


SiOg  53.01 

AlaO., '    30.04 

Fe,u, I 

FeO 

CaO 12.37 

MgO 1 

IStiaO 4.56 

KsO 

HaO 

Total 99.i)8 

SpG 


II. 


III. 


IV. 


51.;}0  I     47.40  ;     ")1.4n 


31.46 


12  20 
"'5.'33" 

'"'.ib' 
"i.hiyi 


29.74 


1.94 

13.  ,30 

.57 

4.99 

1.56 

1.64 

101.14 


31  94 
Trace 


14.31 
.27 
.85 
.21 
.68 

99.71 
2.7041      2.709 


V. 


31.56 


VI.        VII. 


47.30       49.155 

31.50  I     29.620 

1.85  I       1.152 


2.29 

15.39 

.27 

2.52 

.37 

.40 

100.05 

2.70 


14.88 

.93 

1.22 

.38 

1.80 

99.86 


15.309 

.911 

2.914 

.695 

.730 

100.486 


*Copper-l)eaflnK  Rocks.    Monograph  V,  U.  H.  G.  S.,  p.  438. 

+  The  thairk.s  of  the  writer  aro  hero  teiiden>d  to  Prof.  D(Kl){e  and  to  7«Iessrs.  Blasdale 


and  Palache  for  kind  assistance  In  tuakin;;  analyses. 


I 


THE  A.NORTHOSYTKS  OF  MINNESOTA. 


,he  specitie 
of  the  rock 
e  grammes 
were  2.702. 
ies  so  little 
3nce  of  any 
iferred  that 
,  viz:  labra- 
fragments 
Mineralogy 
I  III  of  the 
as  a  whole 

ther  local - 
point,  was 
nt  vitreous 
i  submitted 
ta,  and  the 
the  table. 
Lty,  though 
:alies  being 
he  correct- 
par  of  this 
analysis* 
ver  Bay  is 
lime  and 
le  feldspar 

e  result  of 
lese  rocks 
o  analyses 


^I. 

VII. 

.ao 

49. 155 

.50 

29.620 

.85 

1.152 

.88 

15.309 

.93 

.911 

.22 

2.914 

.38 

.695 

.80 

.730 

.86 

100.486 

39srs.  Bliiadale 


I.  Theoretical  composition  of  labradorite  Ab^  An.,  the 
latter  being  the  formula  calculated  for  the  constituent  feldspar 
of  the  rock  from  near  Encampment  island,  on  the  basis  of 
optical  measurements. 

II.  Analysis  of  the  same  feldspar. 

III.  Bulk  analysis  of  tlie  rock  from  near  Encampment 
island. 

IV.  Analysis  of  constituent  feldspar  of  the  rock  from  cove 
east  of  Split  rock  ]K)int. 

V.  Analysis  of  the  rock  from  point  two  miles  below  Beaver 
Bay  as  given  by  Irving.  Copper  Bearing  Kocks,  page  438. 

VI  and  VII.  Two  Analyses  of  bytownite,  ([uoted  by  Teall, 
British  Petrography.  i)age  146. 

The  name  AnorthoHyU' : — It  is  thus  clear  that  the  mineral ogical 
composition  of  the  rock  is  not  sti'ictly  uniform  and  that  both  the 
names  which  have  been  applied  to  it  as  a  rock  formation  are  un- 
fortunate. Neither  •' anorthite  rock  "  nor  "  labradorite  rock" 
is  a  correct  designation  for  the  rock  as  a  whole;  and  if  a  more 
satisfactory  and  comprehensive  designation  can  be  found,  both 
of  these  terms  should  be  dropped.  The  essential  and  constant 
feature  of  the  rock  is  that  it  is  almost  exclusively  composed 
of  an  allotriomorphic  granular  aggregate  of  basic  i)lagioclase. 
Such  rocks  are  not  unlcnown  to  the  writer.  Large  areas  of 
them  occur  in  eastern  Canada,  and  he  has  met  them  in  the 
field  at  Narodal  in  Norway,  and  in  parts  of  northern  New 
Jersey,  and  in  the  Rainy  Lake  region.  Such  rocks  have  not 
yet.  however,  received  a  satisfactory  place  in  petrograhical 
classifications.  While  considering  the  question  of  a  designa- 
tion for  such  rocks,  the  writer  had  occasion  to  consult  with 
Prof.  Frank  Adams  of  McGill  College,  Montreal,  who,  from 
his  long  study  of  similar  rocks  in  the  so-called  Norian  series 
of  Quebec,  is  probably  our  best  authority  in  this  branch  of 
petrographical  science.  In  this  correspondence  Prof.  Adams 
informs  the  writer  that  he  has  now  in  the  x^ress  a  memoir  on 
just  such  rocks  as  form  the  subject  of  this  paper,  treating 
them  historically,  petrographically,  geologically  and  compara- 
tively. In  this  memoir  Prof.  Adams  will  retain  the  old  name 
"  anorthosite"  for  the  class  of  granular  rocks  composed  of 
plagioclase  to  the  i)ractical  exclusion  of  the  ferromagnesian 
silicates.  Such  rocks  are  regarded  by  him  as  being  at  one 
end  of  the  gabbro  series,  while  at  the  other  end  are  those 
granular  rocks  composed  of  ferromagnesian  silicates  almost 
to  the  exclusion  of  the  ijlagioclases.    The  term  anorthosyte 


1 


8 


BULLETIN    NO.    VIIL 


is  therefore  here  adopted  for  the  plagioclase  rocks  of  the 
Minnesota  coast,  in  accordance  with  the  usage  which  will 
certainly  be  established  by  the  publication  of  Prof.  Adams' 
memoir. 

Accessory  Const Uuents  of  the  Anorthosyte:— The  only  original 
mineral  which  can  be  discovered  in  the  slides  as  a  constituent 
of  the  granular  aggregate,  is  a  faint  violet  brown  monoclinic 
pyroxene.  It  shows  none  of  the  lamellar  structure  of  diallage, 
and  is  evidently  a  feebly  ferriferous  augite.  This  augite,  as 
has  been  stated,  plays  a  very  subordinate  role,  and  is  found  in 
two  modes  of  occurrence  which  are  not  sharply  separable: 

(1)  As  minute  irregularly  shaped  patches,  usually  some- 
what triangular,  filling  a  few  interstital  places  between  the 
large  grains  of  plagioclase.  These  particles  are  usually  of 
uniform  orientation  throughout  and  consist  of  a  single  indi- 
vidual crystal.  (2)  As  more  or  less  rounded  or  bleb-like  in- 
clusions within  the  plagioclase  crystals.  In  the  latter  case  the 
plagioclase  sometimes  shows,  by  its  undulatory  extinction  in 
polarized  light,  an  area  of  molecular  tension  encircling  the 
crystal  of  augite.  In  both  occurrences  the  augite  may  be 
charged  with  magnetite  dust,  but  is  often  seen  free  from  such 
inclusions.  In  some  slides  the  augite  is  perfectly  fresh  and 
shows  no  decomposition  whatever.  In  other  cases  there  is 
observable  a  fibrous  structure  occupying  a  peripheral  zone  of 
varying  width,  the  fibers  being  all  parallel.  In  still  other 
cases  the  augite  has  become  greenish,  due  to  the  presence  of 
ftocculent  chlorite,  and  in  others  the  whole  of  the  augite  has 
passed  over  into  a  nest  of  scaly  chlorite  with,  perhaps,  some 
serpentine.  In  one  instance  a  shred  of  green  hornblende  was 
observed  on  the  periphery  of  a  partially  decomposed  grain  of 
augite,  and  is  probably  also  a  secondary  product. 

Interpositions: — The  inclusions  or  interpositions  in  the  plagio- 
clase are  of  three  general  kinds:  (1)  Original  mineral  in- 
clusion arranged  in  plates  or  rods  parallel  to  definite  crystal - 
lographic  planes.  (2)  Original  liquid  and  dust-like  inclusions 
arranged  in  irregularly  curving  planes  without  reference  to 
the  crystal  structure.  (3)  Secondary  inclusions  of  red  iron 
oxide  in  minute  specks,  arranged  peripherally  to  the  plagio- 
clase or  along  the  cracks  which  occasionally  traverse  it. 

The  interpositions  of  the  first  class  illustrate  in  a  very  strik- 
ing way  the  same  phenomenon  that  has  been  described  by 
Judd  as  due  to  the  process  of  Schillerization.  ( See  plate  II, 
Fig.  1).     The  mineral  plates  when  seen  on  edge  are  of  nearly 


.0- 


ks  of  the 
i^hich  will 
•f.  Adams' 

^  original 
onstituent 
nonoclinic 
f  diallage, 
augite,  as 
i  found  in 
rable : 
i,lly  some- 
tween  the 
isually  of 
ttgle  indi- 
eb-like  in- 
r  case  the 
inction  in 
rcling  the 
i  may  be 
rom  such 
fresh  and 

there  is 
al  zone  of 
till  other 
esence  of 
bugite  has 
aps,  some 
Lende  was 

grain  of 


le  plagio- 
Lueral  in- 
3  crystal- 
nclusions 
jrence  to 
red  iron 
3  plagio- 
it. 

3ry  strik- 
xibed  by 
I  plate  II, 
)f  nearly 


'''"^■'■'••-  "•  '•■'<■■  '•  ^lirr..|,li..l.,^u,,,|,  ,,t  -..,•11,-11  of 
;umr<li,,sy|,.  f,oiii  i,,.;,,-  iMi.ami  ..ihm  I  \-\:nn\.  -howin- 
«'ri«iiiiil    illt.Tiiosilioi.s   o|-  iiiiyii..    in    1:,  I  .riMlMii  t,-.  ''s 


'''-\"-  "•  '•'<'•  -•  H:m(|.(l  iiiiurtliosyK-  at  111.-  ,,.vc 
l.fl.nv  Split-rock  I'oi„(,  Mi^n(^s(,ti,  roast,  l.alu-  Sii  [..Tior. 
Two  .IxU.s  of  (liahiiHc  cut  the  ha.MJ.'d  auo.  (Iiosy t.'  an.! 
ai)i>t'ai-   lo    th.>    left    in    the    i  1 1  iisl  rat  ion. 


(  ill  i|  .     \  \  li    \   \  I   •!  I  I-  I     M    K'.    >I  I  \.\ 


I  il    III    I  I  \    \m    •«        I  'I    \  II     II 


I 


THE   ANORTHOSYTES  OF   MINNESOTA. 


-* 


uniform  thickness  but  of  varying  ien^th.  They  iu-»(  not  present 
in  all  slides,  but  when  they  occur  they  ure  prominent  features 
of  the  section.  They  aj»pear  in  all  cases  to  lie  parallel  to  the 
plane  of  the  brachypinacoid,  and  thus  between  crossed  nicols 
they  a))pear  in  soctifnis  transvtu'se  to  x  P  i  as  h<uivy  bars  or 
dashes  arrau^'ed  parallel  to  the  twinninjC  lamella*  of  the  alblte 
law.  In  sections  but  slijifhtly  inclined  to  r  P  i  llu?  broad  sides 
of  the  plates  may  be  observed;  but  the  breadth  when  thus 
seen  is  se^feral  times  less  than  the  length.  The  plates  are 
lenticular  in  form.  Mineralo^ically  they  api)ear  to  be  identical 
with  the  augit(!  above  described.  They  may  be  com{)ar«>d  in 
the  same  slide  with  the  augite  which  occurs  intei'stitially  be- 
tween the  feldspars,  and  no  essential  difference  can  be  detected, 
the  color  and  refractive  power  being  the  same.  In  many  cases, 
however,  a  single  mineral  {ilate  appears  to  be  mad«!  n\)  of  an 
aggregate  of  minute;  granules  of  augitc.  The  plates  are  also 
frequently  charged  with  granules  of  magnetite.  The  relation 
of  the  plates  to  the  plagioilaso  is  such  as  to  suggest  that  they 
are  original  inclusions,  the  feldspar  being  quite  fresh  and  the 
contact  of  the  plates  with  plagioclase,  when  studied  with  high 
powers,  being  irregular  in  detail.  It  thus  appears  from  their 
mineralogical  character,  and  from  the  mode  of  their  iiiclosure 
that  they  ditfer  from  schillorization  products;  and  that  they 
are  original  inclusions  although  presenting  precisely  the  same 
appearance  under  the  microscope  as  the  schillerized  minerals 
described  by  Judd. 

The  inclusions  of  the  second  class  are  abundant  but  usually 
very  minute.  They  may  be  observed  frequently  congregated 
in  crescent-shaped  areas  and  in  these  cases  the  inclusions  com- 
monly are  pear-shaped.  Bubbles  may  rarely  be  seen  but  they 
are  not  apparently  mobile.  In  almost  all  cases  where  these  in- 
clusions appear  to  traverse  a  crystal  in  bead  like  rows,  these 
rows  may.  by  careful  focusing,  be  seen  to  be  but  the  traces  of 
planes.  These  planes  are  usually  irregularly  curved  and  in- 
tersect one  another  in  all  directions. 

The  inclusions  of  the  third  class  are  abundant  only  locally, 
and  then  they  give  a  faint  reddish  tinge  to  the  rock  as  may  be 
seen  in  sjiecimens  from  Carlton  peak.  They  appear  clearly  to 
owe  their  origin  to  percolating  media,  since  they  are  formed 
only  on  the  periphery  of  the  feldspar  between  neighboring 
grains,  and  along  the  cracks  which  traverse  them.  The  color 
is  a  bright  red  and  the  inclusion  is  usually  a  minute  irregular 
patch. 


10 


BULLETIN    NO.    VIIL 


r, 

I 


DISTRIBUTION    AND    MODE    OK    OCCUHUENCE    OK    THE 

AN0RTH08YTE. 

Two  Modes  of  Occurrence: — Anorthosyte  has  been  observed  by 
tlie  writer  at  several  localities  on  the  Minnesota  coast  between 
Encampment  island  and  Carlton  ptsak,  a  distance  of  46  miles. 
Its  occurrence  seems  to  be  limited  to  the  middle  third  of  the 
<'oast.  The  re<?ion  inland  from  the  coast  is,  however,  largely 
a  /'//•«  incotjnUa,  and  tht.  formation  may  have  an  extensive  dis- 
tribution in  this  region.  Its  occurrences  are  of  two  kinds.  (1) 
Outcrops  of  ridges  and  rounded  surfaces,  in  situ.  (2)  In  the 
form  of  very  abundant  inclusions  as  boulders  and  blocks  im- 
bedded in  the  Keweenian  lava  Hows.  Both  VVinchell  and  Irving 
have  referred  to  some  of  the  localities  where  the  rock  is  found, 
but  as  the  writer  is  able  to  add  to  th<.^  list  of  these  localities, 
and  to  give  a  somewhat  fuller  account  of  the  mode  of  occur- 
rence, he  will  give  a  brief  statement  of  what  is  known  in  this 
regard,  even  at  the  risk  of  repeating  earlier  observations. 

Occurrence  Near  Encampiinnif  Is.'a n -I :— hi  passing  down  the 
Minnesota  coast  from  Duhith,  the  first  locality  at  which  the 
anorthosyte  is  met  with,  is  at  a  cove  on  the  shore  about  half  a 
mile  below  Encampment  island,  [)robably  in  Sec.  6,  Tp.  53,  R. 
IX.  The  anorthosyte  here  occurs  in  the  form  of  very  numer- 
ous boulders  and  angular  blocks  imbedded  in  an  amygdaloidal 
diabase  porphyry te.  which  forms  one  of  the  Keweenian  flows. 
Many  of  the  boulders  ai'e  of  immense  size,  and  several  project 
for  more  tlian  half  their  dimensions  from  the  matrix  in  which 
they  lie.  Plate  I.  Figs.  1  and  '2.  will  give  some  idea  of  the 
aspect  of  these  huge  masses.  Two  of  these  boulders  were 
found  to  measure  13x8  feet,  and  18x0  feet  in  greatest  cross- 
section.  Besides  the  large  masses,  there  are  very  many  small 
boulders  and  blocks  ranging  down  to  about  three  inches  in 
diameter.  Some  of  the  smaller  blocks  are  evidently  detached 
fragments  of  the  larger  ones;  and  sevei-al  of  the  latter  were 
seen  which  had  been  cracked  and  the  fissures  filled  with  dia- 
base-porphyryte.  The  boulders  are  perfectly  fresh,  and  their 
contact  with  the  imbedding  matrix  may  be  observed  with  knife- 
edge  precision.  The  rock  surface  where  the  boulders  are 
enclosed,  has  been  glaciated  and  is  furrowed  with  deep  grooves. 
The  projection  of  the  boulders  four  feet  above  the  general  sur- 
face of  the  country  rock,  both  being  grooved,  is  instructive 
evidence  of  the  limitati(ms  of  the  erosive  power  of  glacier 
action. 


THE 


1 


served  by 
)  between 
46  miles, 
•d  of  the 
r,  largely 
nsive  dis- 
inds.  (1) 
>)  In  the 
ilocks  im- 
nd  Irving 

is  found, 
localities, 
of  occur- 
cvn  in  this 
ions. 

down  the 
which  the 
3ut  half  a 
Pp.  53,  R. 
ry  numer- 
^gdaloidal 
lian  flows, 
■al  project 
c  in  which 
ea  of  the 
iers  were 
est  cross- 
lany  small 

inches  in 

detached 
itter  were 
L  with  dia- 

and  their 
vith  knife- 
Lilders  are 
p  grooves. 
3neral  sur- 
nstructive 
of  glacier 


y 


^ 


I 

I:. 


rS; 


1 


is 


THE   ANOliTHOSYTES   OF   MINNESOTA. 


11 


Vicinity  of  Split-rock  point: — The  second  locality  where  the 
anorthosyte  forms  a  prominent  feature  of  the  geology  of  the 
coast,  is  at  Split-rock  point,  and  at  the  bottom  of  the  small  cove 
immediately  below  the  point.  Here  it  is  found  in  both  the 
modes  of  occurrence  above  mentioned.  The  extremity  of  Split- 
rock  point  presents  a  very  remarkable  aspect.  It  is  a  sheer 
cliff  rising  vertically'  fifty  or  sixty  feet  above  the  surface  of  the 
water.  This  wall  of  rock  affords  a  clean  section  of  a  gi^eat 
breccia.  The  volcanic  rock  which  forms  the  point,  a  diaba.se- 
porphyryte,  is  studded  with  innumerable  boulders  and  angular 
blocks  of  anorthosyte.  The  matrix  being  very  dark  and  the 
included  blocks  whitish  or  yellowish,  the  contrast  is  pro- 
nounced. The  size  of  the  bloclcs  ranges  from  a  few  inches  to 
several  feet  in  diameter.  They  are  in  some  places  congregated 
together  and  in  others  they  lie  isolated  in  the  matrix. 

Around  the  point  to  the  east  we  find  the  anortho.syte  in  place 
on  both  sides  of  the  cove.  On  the  west  side  a  mass  a  hundi-ed 
yards  or  more  in  extent  is  exposed,  and  the  diabase-porphyry te 
is  clearly  seen,  both  to  traverse  it  in  sharply  defined  dykes, 
and  to  mantle  its  upper  surface  in  the  form  of  a  flow.  Here 
also  may  be  observed  a  structural  and  mineralogical  diiforon- 
tiation  of  the  anorthosyte,  which  is  apparently  quite  local  in 
its  character,  and  which  has  been  observed  by  the  writer  only 
at  one  other  exposure.  This  ditTerentiation  is  apparent  in  the 
form  of  dark  bands,  which  traverse  the  mass  and  give  it  a  bed- 
ded aspect,  as  is  exhibited  in  the  illustration.  Plate  II,  Fig.  2. 
These  bands  are  usually  only  a  few  inches  thick,  but  may  some- 
times be  as  much  as  a  foot  across.  They  have  at  this  locality 
a  characteristic  jagged  appeai-ance,  owing  to  the  fact  that  the 
band  has  frequently  no  simple  line  of  demarkation  separating 
it  from  the  normal  anorthosyte,  but  is  interlocked  with  it  by 
means  of  sharp,  wedge-like  prolongations  or  tongues,  which 
are  approximately  parallel  to  the  trend  of  the  band.  The  dip 
of  the  planes  of  the  banding  is  to  the  north-east  at  an  angle  of 
about  sixty  to  seventy  live  degrees.  The  banded  rock  is  tra- 
versed by  three  small  dykes  of  the  diabase-porphyryte  which 
have  a  nearly  vertical  attitude,  and  H'hicli  are  easily  discrimi- 
nated from  the  bands  on  the  ground,  although  they  do  not  ap- 
pear very  differently  in  the  photograph  from  which  the  plate 
has  been  prepared.  These  bands  are  mineralogically  different 
from  the  normal  character  of  the  main  mass  of  the  anorthosyte 
in  having  a  considerable  proportion  of  a  dark  ferro-magnesian 
constituent  ( augite)  which  is  more  or  less  decom]iosed.     Under 


12 


BULLETIN   NO.    VIIL 


the  microiif;ope,  the  rock  which  composes  the  bands  is  not 
otherwise  d liferent  from  the  normal  anortlisyte,  and  the  plagio- 
clase  appears  to  be  in  no  way  affected  cliemically  by  the  de- 
composition of  the  augite,  although  the  rocli  exhibits  a  tend- 
ency to  mechanical  disintegration  by  the  separation  of  the  feld- 
spar grains. 

On  the  other  side  of  the  cove,  secton  6,  township  54,  range 
8,  the  anorthsyte  is  exposed  in  much  greater  mass.  Here  it 
emerges  from  beneath  the  Keweenian  eruptives  and  stands  out  as 
a  bold,  bare  ridge  v/ith  somewhat  rounded  outline,  rising  to  an 
elevation  of  about  200  feet.  The  section  here  presented  is 
shown  diagramatically  in  the  sketch  Pig.  1.  The  base  of  the 
anorthosyte  ridge  is  lianked  to  the  southeast  by  a  mass  of  red, 
'ine  grained  amygdaloldal  fieldspar-porphyry;  and  this  in  turn 
■s  flanked  lake  ward  by  an  amygdaloldal  diabase  porphyry  te,  In 
A'hich  lies  imbedded  a  boulder  of  the  anorthosyte. 

Irving's  views  on  the  occurrence  at  Split-rock: — Irving's  general 
note  on  the  occurrence  in  the  vicinity  of  Split  rock  point  indi- 
cates that  he  regarded  the  anorthosyte  as  an  eruptive  mass, 
although  his  own  observations  are  in  part  quite  inconsistent 
with  this  view. 

He  says:*  "Near  the  middle  of  S.  E.  ^  of  Sec.  5,  township 
54,  range  8  east,  this  gabbro  (one  of  the  Keweenian  flows)  Is 
interrupted  by  a  vertically  placed  mass  of  excessively  coarse- 
grained anor trite  rock  [anorthosyte].  The  cutting  mass  is 
from  50  to  75  feet  wide  and  bears  north  and  south.  It  shows 
on  both  sides  of  a  little  square-angled,  rock  walled  bay,  on  the 
south  point  of  which  it  rises  as  much  as  a  hundred  feet  above 
the  lake.  On  both  sides  of  the  cutting  mass  the  black  gabbro 
is  filled  with  large  angular  masses  of  the  same  coarse  anorthite 
rock.  The  included  masses  sometimes  reach  many  tons  in 
weight,  and  in  some  places  predominate  over  the  including 
gabbro,  which  then  appears  as  if  veining  the  coarser  rock. 
At  the  west  angle  of  the  bay  the  included  masses  are  nearly  absent 
and  the  gabbro  resumes  its  usual  vertically  columnar  appear^ 
ance.  At  the  north  angle  of  the  bay  the  anorthite  rock  rises 
again  to  a  hight  of  over  150  feet.  The  inclusions  of  angular 
masses  of  the  anorthite  rock  in  the  gabbro  indicate  the  more 
recent  origin  of  the  latter,  and  this  conclusion  is  borne  out  by 
the  section  made  from  a  specimen  taken   at.  the   contact  with 


♦Copper  bearlnti  Uocks  MonojJiraph  V  U.  S.  G.  8,  page  302, 


S  a 


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-15 


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X 


THE  ANORTHOSYTES  OF  MINNESOTA. 


13 


the  gabbro.  in  which  relatively  fine  gai)bro  surrounds  the  ends 
of  the  anorthite  crystals  as  the  Ixiso  o'  any  porphyry  does  the 
porphyritic  crystals  which  lie  imboddea  in  it." 

This  locality  is  a  dilTerent  one  from  that  described  by  the 
writer  as  the  cove  immediately  below  Split-i'ock  point,  but  the 
general  conditions  are  the  same.  Notwithstanding  the  evid- 
ence of  the  included  masses,  Irving  seems  to  have  clearly  re- 
garded the  anorthosyte  as  eruptive  thnnigh  the  gabbro.  which 
is  probably  the  same  geologically  with  the  rock  which  the 
writer  has  recognized  as  a  diabase  porphyryte.  For  he  saya 
in  another  place*  "This  anorthite-rock  presents  very  interest- 
ing occurences,  as  described  in  a  subsequent  chapter.  It  ap- 
pears both  as  masses  cutting  black  gabbro,  and  as  included 
angular  masses  in  the  same  rock'.'  This  conclusion  as  to  the 
eruptive  character  of  the  anorthosyte  appears  to  the  writer  to 
be  an  error  due  to  hasty  or  imperfect  observation.  Irving  no- 
where cites  evidence  of  its  eruptive  character.  V)eyond  that  it 
appears  suddenly  in  the  midst  of  his  Koweenian  tlows,  a  state- 
ment which  is  quite  true,  but  which  is,  of  course,  susceptible 
of  a  totally  different  explanation.  There  is  positive,  observ- 
able evidence  of  the  Keweenian  tlows  resting  on  the  surface  of 
the  anorthosyte  and  being  traversed  by  dykes  of  the  same  ma- 
terial as  the  Hows.  This  fact  taken  together  with  the  ab\md 
ant  inclusions  of  the  anorthosyte  fragments,  which  arc  not 
always  angular  blocks,  but  frequently  rounded  boulders,  leaves 
no  doubt  whatever  as  to  the  relative  age  of  the  formations. 

WinchelVs  vieios:—Pvot  N.  H.  Winchell's  views  as  to  the  oc- 
curences at  Split- rock  may  be  gathered  from  the  following  ob- 
servations taken  from  his  notes  published  in  the  ninth  and 
tenth  annual  reports:  -'The  west  side  of  Split- rock  river  at 
its  entrance  to  the  lake  is  low,  but  the  east  side,  or  north-east- 
erly, is  high,  and  formed  of  a  basaltic  bluff  or  rock  which  ap- 
pears on  the  immediate  coast  at  a  short  distance  east  of  the 
river.  It  there  embraces  a  large  block  of  a  whitish-looking 
rock,  which  at  a  distance  appears  to  be  a  granite,  but  which  in 
reality  is  what  has  been  described  by  Norwood  as  feldspar  pro 
trading  through  greenstone.  This  does  stand  up  like  a  dyke, 
but  is  in  reality  older  than  the  trap,  and  occurs  generally  fur- 
ther inland,  forming  hills  several  hundred  feet  high.  This 
bluff  rises  sheer  from  the  water  13()  feet  and  has  basaltic  dark 
trap  on  each  side  of  it,  the  rock  itself  being  massive.     On  the 

*Op.  cit.  p.  .59.  ' 


14 


HULLETIN    NO,    Vlll. 


east  sid(3  of  this  high  rock  the  trap  shows  inchided  masses  of 
the  same  rock,  a  fact  which  Norwood  mentions,  but  yet  speaks 
of  the  feldspar  as  a  protuded  mass  of  later  date  than  the  trap."* 

AiiorthoHijfe  (Jon founded  loith  thv  Kcweenian  Eruptires: — It  will 
thus  be  seen  that  Winchell  correctly  interpreted  the  pheno- 
mena as  indicating  that  the  anorlhosyte  is  an  older  formation 
than  the  eruptive  with  which  it  is  associated.  But  he  still 
regarded  the  anorthosyte  as  a  member  of  the  Cux)riferous  or 
Keweenian  series.  For,  in  the  report  for  the  following  year 
he  gives  the  following  formulation  of  one  of  his  conclusions: 
"The  feldspar  masses  are  of  the  same  rock  (geologically)  as 
the  Rice  Point  gabbro  (V),  and  both  are  the  result  of  copious 
and  perhaps  one  of  the  earliest,  igneous  outflows  of  the  Cupri- 
ferous. The  more  copious  the  igneous  outllow,  the  coarser 
the  resulting  crystalli^iation  and  the  higher  the  hills  formed, 
as  well  as  the  purer  the  labradorite  material.  The  later  out- 
flows derived  fragments  from  the  "clinker  fields"  and  from  the 
knobs  of  feldspar  already  formed,  as  they  passed  along  f 

Thus  none  of  the  earlier  observers,  Norwood,  Winchell  or 
Irving,  has  differentiated  the  anorthosyte  masses  exposed  at 
Split- rock  from  the  general  aggregation  of  volcanic  flows  which 
constitute  the  Keweenian  series  on  the  Minnesota  coast. 

Occurrences  at  Beaver  Buy: — At  the  next  high  point  on  the 
shore  to  the  east  of  Split-rock  point  the  same  conditions,  as  far 
as  regards  the  inclusion  of  masses  of  anorthosyte  in  the  dark 
lavas  is  repeated;  but  it  is  not  till  Beaver  Bay  is  reached  that  the 
anorthosyte  is  again  exposed  in  place.  Here,  it  may  again  be 
clearly  observed  in  clean,  wave-washed  cliff  sections  to  be  the 
basement  upon  which  the  lavas  were  extravasated,  and  to  have 
here  also  afforded  numerous  boulders  and  blocks  which  were 
caught  up  and  enclosed  in  the  lavas.  All  former  notes  as  to 
the  occurrence  of  anorthosyte  at  Beaver  Bay  are  singularly  de- 
ficient in  geological  information.  Irving  enters  in  some  detail 
into  an  account  of  the  geology  of  Beaver  Bay,  and  publishes  a 
special  sketch  map  to  show  the  distribution  of  the  various 
kinds  of  rock,  but  he  barely  alludes  to  the  anoi'thosyte.j 
Wincheirs  most  important  note  is  as  follows:  "Between 
Beaver  Bay  and  the  Great  Palisades  are  numerous  feldspar 
masses,  in  the  coast  series,  and  inland  from  the  shore,  a  very 
short  distance,  is  a  range  of  low  hills  made  up  of  feldspar  with 
trap  rock  on  the  flanks.  "§ 


♦Ninth  Annual  Ueport  Geol.  and  Nat.  Hist. 
+Tenth  Annual  Report  Geol,  and  Nat.  Hist. 


JCopper-bea-'lnK  Rocks,  pp.  30(1,  307. 
tSNtnth  Annual  Report  Geol.  and  Nat. 


Survey  Minn..  1880,  p.  30. 
Survey  Minn.,  1881,  p.  114. 


Hist.  Survey  of  Minn.,  1880  p.  ;i4. 


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THK    ANOUTm)SYTKS   UF    MINNESOTA. 


15 


I 


^m  -'' 


At  Beaver  Ray  there  ar(>  four  separate  and  distinct  occurrence* 
of  the  anorthosyte.     The  first  of  tliese  is  at  tlie  shiiijjrlt'  ''nvo  at 
tho  extremity  of  the  soutli  west  lieudland  of  IJeaver  buy.   and 
on   the  south  sid(«  of  the  vortical  cliff  of  nid  porjihyry  which 
hero  forms  so  striking  a  landnnirk.     The  anorthosyte  (xtcupies 
at  this  place  an  an»a  of  about  (KiO  l)y  iJOO  feet.     'J^he  greater 
part  of  the  mass,  which  rises  to  about  30  feet  above  the  level 
of  the   laU(!.    is   the    normal    coarse  wliitish   anorthosyte    free 
from  any  special  structural  features.     On  the  side  facing  the 
shingle  cove,  however,  i.  e..  at  the  north-eaHtern  extremity  of 
tho  mass,  a  pronounced  banded  structure  is  apparent,  resemb- 
ling som(^what   that   d(?seribed    in    the    vicinity  of    Si)litrock 
point.    The  stratiform  apftearanceof  themass  is  well   illustra- 
ted in  Plate  III.      The  di[)  is  to  the  southeast.    Tlu;  stratiform 
structure  is  here  duo  to  the  fact  that  there  are  certain  sheet-like 
layei's  somewhat  richer  in  pyroxene  than  the  rest  of  the  rock. 
Tho  decomposition  of  tho  pyroxene  has  tho  effect  of  staining 
tho  hiyer  so  atlected  a  yellowish  or  greenish  rusty  color,  so 
that  it  presents  a  strong  contrast  with  tho  unal'focted  portions 
of  anorthosyte  which  he  between  the  layers.     The  decomposi- 
tion of  the  pyroxene  has  not  chemically  affected  the  consti- 
tuent feldspar  of  the  rock  oxc(»pt  to  a  very  limited  extent,  but 
it  has  the  etToct  of  mechanically  disintegrating  it  so  that  it. 
crumbles  readily  in  the  hand.     Hence,  as  may  bo  soon  in  the 
plate,  the  dark  layers  weatlior  out  as  grooves  (n- depressions. 
Under  the  microscope  the  feldspar  in  specimens  from  tho  dark 
bands   is   fresh,  save  for  a  few  points  whore  decomposition 
products    appear.     It    is.    however,    much    cracked    and    the 
yellowish  decomposition  products  of  the  augite  are  distributed 
along   tho   cracks.     Norwithstanding    thf>    presence  of   those 
cracks  these  feldspars  show  no  strain  phenomena  such  as  un- 
dulatory  extinction,  faulting,  shearing,  cataclastic  structure, 
granulation,     etc.      The    original     allotrioraorphic    granular 
structure  has 'not  boon  disturbed,  and  it  is  highly  improbable 
that  banding  is  in  any  way  associated  with  shearing  action 
after   the   final  solidification   of   the   rock.     It  seems   to  tho 
writer  to  be  essentially  due  to  some  local  chemical  differentia- 
tion, associated  with  movement,  in  tho  thickly  viscous  magma 
prior  to  crystallization.     This  stratiform   facies  of  tho   anor- 
thosyte is  at  this  place  cut  by  a  well  defined  dyke  of  olivine- 
diabase,   which   forms   the   jointed  rock  on  the  right  in   the 
illustration.     Tho  dyke  is  about  25  feet  wide,  is  nearly  vertical 
and  has  a  strike  corresponding  with  the  general  trend  of  the 


16 


BULLETIN   NO.    VIIL 


coast.  The  anorthosyto  is  observable  on  both  sides  of  the 
dyke.  The  latter  has  weathered  out  more  easily  than  the 
anorthosyte,  and  its  place  is  therefore  marked  by  a  depression, 
or  negative  dylie  profile.  In  following  this  depression  south- 
ward the  olivinediabase  of  the  dyke  is  covered  with  great, 
loose  blocks  of  the  anorthosyte  which  have  fallen  from  the 
wall  on  either  side. 

To  the  southward  the  anorthosyte  gives  way  to  a  mass  of 
hypersthene-diabase  in  which  are  enclosed  many  huge  blocks 
of    the    former  rock.       Some  of  the.se  V)loeks  are  20  feet  in 
diameter. 

The  second  occurrence  ol  the  anorthosyte  at  Beaver  bay  is 
on  the  north  east  side  of  the  south  headland  of  the  bay  whe:^ 
it  forms  two  islets,  as  noted  by  Irving' 

The  ihird  occurrence  is  on  the  nrjrth  west  shore  of  the  bay, 
extending  from  the  base  of  the  sand  spit  for  over  three  eighths 
of  a  mile  north- eastward.  The  shore  contour  is  serrated  with 
alternating  little  rocky  coves  and  points.  In  the  bottom  of 
nearly  all  the  coves,  as  well  as  on  several  of  the  points,  the 
anorthosyte  maybe  seen  in  rounded  rochcs  )twutonn>'('s  surfaces, 
with  the  diabase  which  predominates  on  the  points  either  rest- 
ing on  it  as  a  mantle,  or  eruptive  through  it  in  the  form  of 
aykes.  The  anorthosyte  is  practically  continuous  from  the 
base  of  the  sand  spit  for  the  distance  above  mentioned.  The 
gracefully  rounded  forms  of  the  old  surface  of  the  anorthosyte 
where  it  passes  underneath  the  Keweenian  flows  is  especially 
interesting  and  is  well  illustrated  in  Plate  IV. 

Another  contact  showing  the  same  relationship  is  illustrated 
in  Plate  V.  The  humraocky  and  roches  moutonnres  aispect  of 
the  old  anorthosyte  surface  is  entirely  analogous  to  the  phe- 
nomena whicli  the  writer  has  elsewhere*  described  as  prevalent 
at  the  contact  of  the  Keweenian  (Nipigion)  and  the  Archaean  in 
other  parts  of  the  lake  Superior  region. 

The  anorthosyte  is  traversed  here  not  only  by  dykes  of  dia- 
base but  also  by  the  common,  red,  acid  eruptive  of  the  country 
(quartz  porphyry  and  granophyre)  which,  in  the  form  of  irreg- 
ular dykes,  intersects  the  older  dykes  of  diabase.  The  diabase 
which  lies  on  the  anorthosyte  holds  many  lartre  blocks  and 
boulders  of  the  latter  of  the  same  character  as  those  i:)re- 
viously  described.     It  is  strongly  amydaloidal  in  places. 

♦Note  on  the  pro-palii-ozoic  surface  of  tlie  Archu'iin  Terranes  of  Canada,  Bull. 
Oeol.  Soe.  Am.  Vol.  1,  pp.  l(ia-174. 


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V 

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t 


THE  ANORTHOSYTES  OF  MINNESOTA. 


17 


The  fourth  occurrence  of  tho  anorthosyte  rock  is  at  the  first 
falls  of  Beaver  river  and  is  described  by  Irving  in  the  follow- 
ing paragraph:  "At  several  points  along  Beaver  river 
this  black  gabbro  carries  large  masses  of  anorthiterock 
I  anorthosyte]  similar  to  that  described  as  occurring  further  up 
the  coast.  The  boulder  like  character  of  the  anorthite-rock, 
though  often  pronounced  is  not  always  so  plain  and  in  some 
places  it  looks  more  like  a  dependency  of  the  prevailing  black 
gabbro.*  A  careful  inspection  of  the  occurrence  by  the 
writer  enables  him  to  state  that  there  is  no  reasonable  doubt 
whatever,  of  the  foreign  and  included  character  of  the  anor- 
thorsyte  boulders 

Shore  below  Bearer  liaij  : — The  next  locality  down  the  coast 
where  the  anorthosyte  appears  is  at  a  point  about  a  mile  and  a 
half  below  the  north  east  headland  of  Beaver  Bay  and  directly 
opposite  a  small  island  which  lies  about  a  quarter  of  a  mile  off 
shore.  Here  the  anorthosyte  emerges  from  beneath  the 
sheeted  trap  rocks,  and  has  an  exposure  of  over  one  hundred 
yards  in  the  form  of  bold,  yellowish- white  domes.  It  also 
occupies  the  island  above  mentioned  which  is  about  one- eighth 
of  a  mile  long.  The  island  is  traversed  by  two  vertical  dykes 
having  a  strike  transverse  to  the  general  trend  of  the  shore. 
One  of  these  dykes  is  200  feet  wide  and  is  of  the  character  of 
a  diabase  or  gabbro.  very  dark  in  color,  coarse  in  texture  and 
rich  in  iron  ore.  The  second  dyke  is  quite  small  and  may  be 
simply  an  apophysis  of  the  larger  one.  The  contact  of  the 
dykes  with  the  anorthosyte  which  occupies  the  mass  of  the 
island  is  sharply  observable  as  a  vertical  plane  with  the  intru- 
sive rock  somewhat  finer  grained  near  the  dyke  wall  than  at 
points  near  the  middle.  A  quarter  of  a  mile  further  down  the 
shore  the  anorthosyte  again  appears  in  an  exposure  several 
hundred  yards  in  extent  and  is  here  also  clearly  intersected  by 
dykes  of  the  dark  diabase. 

Half  a  mile  further  on  at  a  point  on  the  shore  about  three- 
quarters  of  a  mile  beyond  the  east  end  of  the  island  above  men- 
tioned, the  anorthosyte  is  again  exposed;  and  here,  just  as  at 
Beaver  Bay,  the  domed  surface  of  the  old  pre-Keweeniau  ter- 
rane  is  seen  to  be  capped  with  a  sheet  of  amygdaloidal  diabase 
which  adjusts  itself  to  the  curvature  like  a  mantle. 

Irving's  observations  on  these  exposures  arc  summarized  in 
the  following  note:  "At  several  points  in  this  vicinity  the 
black  rock  was  observed  to  include  masses  of  coarse  anorthite- 


♦Oopper-bearinK  Rocks,  p.  306. 
S 


18 


HULLETIN    NO.    VIII. 


rock.  Tho  latter  did  not  appear  to  occur  here'  in  boulder-like 
masses,  but  rather  in  irregular  outlined  areas.  At  one  point 
on  the  shore  of  Sec.  6,  directly  west  of  the  island  above  referred 
to,  the  white,  anorthite-rock  rises  like,  a  dome  in  the  black  gabbyo, 
which  is  seen  above  it  and  on  both  sides  of  it*  The  southern  point 
of  the  island  is  formed  of  anorthite-rock;  and  due  north  from 
this  point,  on  the  mainland,  is  another  area  of  white  rock  ap- 
parently trending  north  and  south."  It  is  difficult  to  under- 
stand why  Irving  failed  to  api)reciate  the  fact  that  the  anorth- 
osyte  areas  represent  the  eroded  surface  of  a  pre- Keweenian 
terraue. 

Baptism  River:  The  next  locality  where  this  interesting  for- 
mation appears  is  on  the  Bajitism  river,  about  half  a  mile  up 
stream  from  its  mouth.  In  the  vicinity  of  the  foot-bridge, 
which  here  spans  the  rocky  gorge  of  the  stream,  the  anortho- 
syte  is  observable  both  in  large  masses  not  fully  exposed,  and 
in  the  'form  of  boulders  several  feet  in  diameter  imbedded  in 
vesicular  and  amygdaloidal  lava. 

On  the  Slope  of  Saw-teeth: — Farther  along  the  coast  about  two 
miles  and  a  half  below  the  mouth  of  Baptism  river,  the  anor- 
thosyte  again  crops  out  on  the  summit  of  a  forest-clad  hill 
about  half  a  mile  inland  from  the  lake,  and  800  feet  above  the 
level  of  its  surface.  The  exposure  has  an  extent  of  100  by  25 
feet  and  its  relations  to  the  flanking  eruptives  are  not  revealed. 
Carlton  Peak: — The  next  occurrence  of  the  anorthosyte  is  at 
Carlton  peak,  a  little  below  the  Temperance  river.  This  ex- 
posure is  the  most  extensive  and  jjrobably  the  most  important 
of  any  on  the  coast.  This  fact  that  the  summit  of  the  peak  was 
composed  of  this  rock  was  tirst  announced  by  Norwoodf  who 
stated  that  it  was  "composed  entirely  of  feldspar  rock"  but 
imagined  that  it  was  a  huge  dyke.  Irving  makes  the  following 
statement  regarding  the  occurrence:  "The  high  bluff  known 
as  Carlton's  peak,  near  Temperance  river,  shows  at  its  summit 
numerous  large  angular  fragments  of  anorthite-rock,  such  as  has 
already  been  described  in  connection  with  the  Beaver  Bay  group. 
None  was  seen  that  could  certainly  be  regarded  as  in  place: 
nevertheless  the  mountain  is  without  much  doubt,  composed 
of  this  rock,  and  I  should  regard  the  rock  as  having  antedat- 
ed the  Temperance  group  flows  rather  than  as  a  cutting  mass." 
It  is  quite  apparent  from  this  statement  that  Irving  himself  did 
not  visit  the  summit  of  Carlton  peak,  and  must  have  based  his 

*Tho  itullcs  are  the  writer'^s. 

+Hcport  of  the  Ueol.  Survey  of  Wisconsin,  lovvu  imd  Miiiricsota  by  I).  1).  Owen  p.  380. 


THE    ANOUTHUSVTES    OK    MINN  LOSOTA. 


1!) 


statement,  as  to  the  occurrence  there,  upon  the  imperfect  ob- 
servations of  others.  The  writer  ascended  the  peak,  and  not 
only  confirmed  Norwood's  statement  that  the  summit  is  com- 
posed of  anorthosyte.  but  also  found  that  the  whole  upper  half 
of  the  mountain  is  made  up  of  the  same  material.  The  expos- 
ure is  an  exceedingly  bold  and  strikinj;  one.  The  peak  is  027 
feet  hi^^h  above  the  lake  and  al)Out  a  mile  and  a  half  distant 
from  the  shore.  It  rises  in  the  form  of  a  ,u'r<'at  dome  and  is  a 
very  conspicuous  land-mark:  the  domed  aspect  bein.ir,  however, 
confined  to  the  upper  part  of  the  mountain.  The  mountain  is 
flanked  on  all  sides  by  the  sheeted  lavas  of  the  Keweenian  and 
these,  by  their  gentle  dip  lake  ward,  ctmstitute  an  easy  grade 
to  the  declivity  of  the  dome,  nearly  half  way  to  the  summit. 
This  gentle  slope  is  heavily  timbered  and  in  part  terraced,  but 
has  been  burnt  in  places;  while  the  upper  half  of  the  mountain 
is  practically  a  continuous  bare  surface  of  anorthosyte,  pre- 
senting to  a  i-emarkoble  degree  of  perfection  the  roches  mou- 
toum'cs  curves  down  to  where  it  plunges  beneath  the  mantle  of 
lavas  which  Hank  the  base.  The  mass  has  none  of  the  charac- 
ters of  a  dyke  or  intrusive  boss  as  Norwood  supposed.  The 
relations  which  obtain  are  illustrated  in  the  accompanying  dia- 
grammatic section  Pig.  -.*  The  area  of  anorthosyte  exposed 
at  Carlton  peak  is  probably  more  than  half  a  mile  square. 
Beyond  Carlton  peak  no  other  exposures  of  anorthosyte  have 
been  observed  down  the  coast,  and  none  are  known  in  Canad- 
ian territory  in  the  region  around  the  lake. 

GEOLOGICAL   RELATIONS   OF   THIC   ANORTHOSYTE. 

Pre-Keweenian  Age: — In  the  foregoing  pages  sufficient  evi- 
dence has  been  set  forth  to  demonstrate  the  pre-Keweenian 
age  of  the  anortliosyte.  This  evidence  is  four  fold.  (1)  The 
anorthosyte  is  traversed  by  dykes  of  the  Keweenian  eruptives 
both  acid  and  basic.  (2 )  The  Keweenian  lavas  hold  imbedded 
in  them  innumerable  boulders  and  blocks  of  anorthosyte  evi- 
dently detached  from  a  pre-existing  terrane.  (3)  The  anor- 
thosyte forms  the  surface  upon  which  the  Keweenian  lavas  now 
rest,  and  upon  which  they  were  originally  extravasated.  (4) 
The  anorthosyte  affords  both  by  its  petrograidiical  character 
and  by  the  nature  of  its  surface  the  most  satisfactory  evidence 
of  profound  erosion  prior  to  the  extravasation  of  the  Keweenian 

eruptives. 

I)0)iH'd  and  ITiiiivnockn  C/ia  racter  of  tin'  Pre-  Keiveenidn  Surface:— 
Not  only  does  the  surface  of  the  anorthosyte  demonstrate  the 
erosion  of  that  formation  in  pre-Keweenian  time,  but  it  shows 


*Puclnsj  patte  l;J. 


20 


UULLKTIN    NO.    VIII. 


that  the  Keweenian  lavas  have  only  to  be  stripped  from  its 
surlace,  by  the  ordinary  forces  of  denudation,  as  has  been  done 
in  several  instances  cited,  in  order  to  att'ord  us  the  typical 
domed,  humniocky,  and  ror/ics  niouto^uieefi  surface  which  is  usu- 
ally ascribed  to  glacial  action.  This  type  of  surface  prevails 
over  the  greater  part  of  the  Archajan  terranes  of  Canada  and 
passes  in  this  peculiar  condition  under  the  base  of  the  palajo- 
zoic  column  wherever  the  contact  has  been  observed.  The 
writer  has  in  a  former  paper  inferred  from  this  fact  that  the 
hummocky  character  of  the  Archjcan  terranes  of  Canada  is  not, 
as  is  commonly  supposed,  ascribable  wholly  to  glacial  action 
in  Pleistocene  time,  but  that  it  is  essentially  pre-pahoozoic  in 
its  origin,  being  only  modified  by  Pleistocene  erosion.*  The 
beautifully  clear  evidence  here  recorded  and  illustrated  (Plate 
IV)  as  to  the  domed  and  roches  nioutonnees  surface  of  the  pre- 
Keweonian  rocks  of  the  Minnesota  coast  where  they  pass  under 
the  Keweenian  lavas,  is  in  harmony  with  the  descriptions  and 
conclusions  given  in  the  paper  referred  to;  and  demonstrates 
that  in  this  regard  the  anorthosytes  have  one  pronounced  fea- 
ture in  common  with  the  Archasan  of  other  parts  of  the  lake 
Superior  region. 

Interval  of  erosion:— The  character  of  the  anorthosyte  as  set 
forth  in  the  petrographical  notes  is  such  as  to  warrant  the  as- 
sumption that  it  is  a  plutonic  formation,  and  that  it  solidified 
in  the  form  in  which  we  now  see  it  under  deep  seated  condi- 
tions; and  that  therefore  the  amount  of  erosion  necessary  to 
reveal  it  at  the  surface  must  have  been  great.  The  interval  in 
which  the  work  of  erosion  was  effected,  was  probably  the  same 
pre- palaeozoic  interval  as  that  which  effected  the  reduction  of 
the  Archaean  to  the  great  hummocky  plane  which  occupies  so 
large  a  part  of  the  North  American  continent,  and  which  re- 
veals at  the  surface  the  great  areas  of  Archajan  granites. 

Absence  of  the  Animikie: — The  recognition  of  a  pre- Keweenian 
terrane  on  the  Minnesota  coast  involves  certain  consequences 
of  geological  importance.  One  of  these  is  that  the  formations 
of  the  Animikie  so  largely  developed  in  the  vicinity  of  Thunder 
bay,  and  there  underlying  the  Keweenian,  are  here  wanting. 
Former  writers  have  assumed  the  existence,  beneath  the  Ke- 
weenian, of  the  Animikie  rocks  along  the  entire  coast.  But 
the  repeated  outcrops  of  the  anorthosyte  at  the  base  of  the  Ke- 
weenian shows  that  for  the  middle  third  of  the  Minnesota  coast, 
at  least,  the  Animikie,  is  wanting.     The  relations  thus  become 

*Pre-Pala>()zoio  Surface  of  the  Arclia-au  Tunancs  of  Canada.    Bull.  Geol.  Soc.  Am., 
Vol.  1,  pp.  163-174. 


THE   ANOHTHOSYTEfS   OK    MINNKSOTA. 


21 


iUO- 


entirely  analogous  to  those  whu-h  obtain  on  the  Canadian  por- 
tion of  tht!  lake  Superior  coast  beyond  Thunder  hay  where  the 
Animikie   is  wanting  and  the  Keweenian  (Nipigon)  rests  di 
rectly  upon  the  humniocky  surface  of  the  Archfean. 

Shallowness  of  the  Keweenian:  —  Another  important  conse- 
quence, resulting  from  a  recognition  of  the  true  character  of 
the  anorthosytes.  is  a  correction  of  Irving's  estimate  of  the 
thickness  of  the  Keweenian.  In  his  account  of  the  stratigraphy 
of  the  Minnesota  coast.  Irving  places  the  thickness  of  the 
Keweenian  series  at  i\),000  feet,  stating  that,  in  all  probability, 
it  may  reach  22,000  or  24.000  feet.*  Disregarding  the  first 
group,  the  St.  Louis  gabbros,  for  the  thickness  of  which  he 
admits  he  can  give  no  good  figures,  he  places  the  thickness  of 
the  remainder  of  the  Keweenian  at  17,000  feet  At  Split-rock 
he  places  the  thickness  at  10. 000  feet,  and  at  Temperance  river 
the  maximum  of  17.000  is  reacthed.  There  is  something  sadly 
astray  with  these  estimates,  and  with  the  stratigraphy  upon 
which  they  are  based.  According  to  Irving  the  Keweenian 
series  on  the  Minnesota  coast  is  thickest  along  its  middle  third. 
It  is  along  tliis  part  of  the  coast  that  the  underlying  basement, 
upon  which  the  Keweenian  rests,  crops  out  sufficiently  abund- 
antly to  demonstrate  that  the  series  is  comparatively  thin, 
ranging  from  zero,  locally,  up  to  a  few  hundred  feet.  The 
Keweenian  on  this  coast  is  by  no  means  the  excessively  thick 
series  that  it  has  been  represented  to  be.  In  tlie  opinion  of 
the  writer,  its  maximum  thickness  is  not  more  than  one- tenth 
of  the  value  at  which  Irving  placed  it.  All  estimates  based 
upon  the  dip  of  the  lava  sheets  are  fallacious,  unless  it  is 
clearly  recognized  that  a  large  jiart  of  the  dip  is  the  original 
slope  of  the  surface  over  which  the  lavas  llowed;  and  that 
there  is  a  constant  and  scarcely  avoidable  danger  of  piling  up 
contemporaneous  flows,  one  on  top  of  another,  by  an  undue 
extension  of  imaginary  stratigraphic  planes.  In  this  way 
figures  are  obtained  foi-  the  thickness  as  enormous  as  they  are 
absurd.  The  stratigraphy  of  volcanic  flows  and  instrusive 
sheets,  traversed  by  many  dykes,  is  not  so  simple  a  problem  as 
Irving  seems  to  have  regarded  it.  and  his  analysis  of  that 
stratigraphy,  his  subdivision  of  the  Keweenian  into  groups, 
and  his  estimates  of  the  thickness  of  the  various  portions  of 
the  series  are  of  little  value;  a  statement  which  it  is  as  painful 
to  make  as  it  is  necessary  in  the  interests  of  sound  geology. 


*C'opi)or-l)oiiiiiig  HoL'ks.  p.  26C. 


22 


liULLKTlN    NO.    VIII. 


Correlation  mid  nnmr  nf  f/ic  foniiafioii: — Petroj^'raphically,  tlie 
anoi'thosytos  of  the  Minnesota  <;oast.  aro  clo.sely  alliod  to  cer- 
tain gi'(!at  uiass(»s  or  areas  of  pla«^ioclase  rocks  which  prevail 
in  the  Province  of  Quebec,  and  which  are  known  al.so  in  the 
Adirondack's  and  in  New  Jersey.  Adams,  who  has  given  many 
years  of  study  to  tlie  (Quebec  occurrences,  has  shown  that  tiiey 
are  irrupt ive  masses  breaking-  throufi^h  the  ordinary  <,'n(>isses 
of  the  Archioan.*  It  woukl  app<>ar,  however,  tliat  they  Ions; 
antedate  the  ad\'ent  of  the  pahoozoic,  that  they  were  phitonic 
intrusives  in  the  Archioan  prior  to  the  great  interval  of  erosion 
which  separated  the  Archioan  and  Pahcozoic,  and  that  their 
appearance  at  the  present  surface  of  that  region  is  duo  to  pre- 
pakooz</ic  denudation.  On  the  hilco  Superior  coast  the  relation 
of  the  anorthosytes  to  the  Archjoan  rocks  is  not  revealed,  but 
it  seems  highly  probable  that  they  are,  here  also.  pre-palo90- 
zoic,  and  were  also  plutonic  intrusives  in  the  Archioan  and  ex- 
posed at  the  pre  paluMjzoic  surface  by  the  sa.ae  erosion  as  that 
which  denuded  the  anorthosyte  areas  of  Quebec.  Moreover 
the  writer  has  elsewheref  noted  the  occurrence  of  plagiocla.se 
rocks  at  Bad  Vermilion  lake,  under  the  name  of  saussurite 
gabbro.  which  are  intrusive  in  part  of  the  Archc'oan.  This 
saussurite  gabbro  is  for  the  mostpart  composed  wholly  of  basic 
plagioclase,  and  differs  petrographically  from  the  Minnesota 
anorthosytes  chielly  in  the  decomposition  of  the  feldspar  into 
the  saussurite  aggregate.  Dykes  of  very  coarse  plagioclase 
rock  (saussuritic)  have  also  been  observed  by  the  writer  tra- 
versing the  Archioan  on  the  international  boundary  at  Otter- 
track  lake,  about  forty  miles  north  of  Carlton  peak.    , 

Fully  aware  of  the  danger  of  establishing  geological  correla- 
tions on  a  petrographical  basis,  the  writer  is  nevertheless  dis- 
posed to  accept  the  suggestion  which  these  petrographical 
analogies  offer,  and  to  enunciate  the  hypothesis  that  the 
anorthosytes  of  the  Minnesota  coast  are  the  geological  equiva- 
lents of  the  anorthosytes  of  Quebec,  i.  e. .  that  they  are 
plutonic  irruptives  invading  the  Archaian,  and  yet  long 
anterior  to  the  Taconic  or  basal  system  of  the  palaeozoic 
rocks.  For  rocks  of  such  a  relationship  the  term  Norian  will 
doubless  be  eventually  retained  in  geological  nomenclature. 
Were  the  correlation  here  suggested  scientifically  demonstra- 
ble the  name  Norian  would  be  a  sufficient  designation  for  the 
geological  formation  composed  of  these  anorthosytes.     Since, 

*Ge<)l()i;lciil  Survey  of  Canada.    Summary  Reports  for  1887  and  1888. 

tOeol.  of  Riliny  T.iikc  Region,  Geol.  Survey  of  Ciinada.  Annual  Report  1888,  pp.  56-57 


ll 

I: 

ii 
r 
i 
I 
t 


THK    ANOIiTHOSVTKK    OF    MINNESOTA. 


28 


however,  the  corn^lation  is  merely  an  liy|)othesi.s.  to  serve  as  a 
basis  for  future  discussions,  a  local  name  for  the  formation 
is  highly  desirable.  The  writer,  therefore  suggests  the 
name  "Carltonian,"  fi'om  Charlton  pea,k,  where  the  formation 
is  most  extensively  exposed,  as  the  geological  designation  of 
the  formation;  and  he  tentatively  (;orrelatef  th<.'  Carltonian 
of  lake  Superior  with  the  Norian  of  the  I'rovince  of  Quebec, 


IL 


TlIK  l.\(V()LITir  SII.LS  OF  TIIK  XOKTIIWIXr 
COAST  OF  LAkF  SFPFIMOH. 


INTRODUCTION. 

General  Nole — Among  the  salient  geological  features  of  the 
northwest  coast  of  lak<^  Superior,  the  most  prominent  are  prob- 
ably the  remarlcable  sheets  of  "trap"  which  are  constantly 
associated  with  the  Animikie  and  Nipigon  groups  of  sediment- 
ary rocks,  between  Pigeon  ri\  er  and  the  Slate  islands.  It  is  to 
the  presence  of  these  trap  sheets  that  the  bold  and  picturesque 
topography  of  Thunder  cape,  McKay's  mountain.  Pie  island. 
Nipigon  bay,  and  the  many  sheer-walled  mesas  and  tilted 
blocks  of  the  region  is  due. 

These  trap  sheets  are  usually  nearly  horizontal,  and  are  of 
great  extent.  They  occur  in  two  ways:  (1)  In  the  form  of 
thick  sheets,  usually  from  ')0  to  200  feet  in  thickness,  (occasion- 
ally 800  or  even  400  feet)  at  the  summit  of  the  local  stratigraph- 
ic  column,  resting  on  the  little  disturbed  shales  and  sand- 
stones, and  constituting  the  surface  of  the  tableland  or  mesa. 
In  geological  descriptions  these  thick  sheets  have  been  re- 
ferred to  as  "trap  caps."  (2)  As  thin  sheets,  usually  from  4  to 
20  feet  in  thickness,  intercalated  with  tVie  shales  and  sandstones 
at  a  much  lower  level  in  the  local  stratigraphic  column. 

These  trap  sheets  are  prevalingly  diabase,  passing  in  places 
into  gabbro,  and  have  been  regarded  by  all  previous  writers  on 
tlie  geology  of  the  region  as  wholly  or  in  part  volcanic  flows. 
The  lower  and  thinner  sheets  have  been  regarded  as  contem- 
poraneous with  the  sedimentary  rocks  with  which  they  are  in- 
tercalated, and  the  capping  sheet  has  been  referred  to  very 
commonly  as  the  "crowning  overflow."  and  by  one  writer  has 
been  designated  the  "gabbro  flood."  These  sheets  are  very 
characteristic  associates  of  the  Animike  group,  and  from  the 
descriptions  which  have  been  given  the  notion  is  current,  both 


I.A(;C()IJTM'    SII.LS. 


25 


in  litorat.iiro  and  amoiij;  iroolo^'isls  who  liav»!  liad  occasion  to 
becotn(>  familiar  with  lalcc  Suporior  <jrnology.  that  the 
Animilcic^  <,'i"oii|)  is,  likf  the  K'lwt'cniaii,  paiMly  volcanic  in  its 
composition. 

I'kir/icrdescriiifioHs  Tliis  idoa  was  first  ciumciatod  by  L<>f?an. 
wiio  in  his  description  of  the  rocks  of  the  north  shore  of  lake 
Suporior  *rofers  tlius  to  the  formations  now  known  as  the 
Animikie  <ri"oup:  ••Bluish  slates' or  shales  intiM'stratifir'd  with 
trap.  *  *  *  *  *.  Ti-aj)  bands  conformable  with  thirst  rati- 
ti(!ation  are  interstratihed  in  several  parts  of  the  vertical 
amount,  but  they  occur  in  greatest  thickness  towards  the  bot- 
tom not  far  from  the  cVierty  beds,  and  at  the  summit  overlying 
the  whoh;  foi-mation.  *  *  *  In  all  cases  it  presents  a  very 
striking  sub-columnar  structure  at  right  an'_rli>s  to  the  plane  of 
the  stratiti(!ation.  and  the  crowning  overflow  gives  a  jjeculiar 
aspect  to  the  whole  region  occupied  by  the  formation  to  which 
it  belongs.'' 

Practically  the  same  language  was  again  used  by  Logan  in 
his  account  of  the  g<'ology  of  ('anada  published  in  180;J. 

Bell  ado|)t(Hl  the  same  view,  that  these  trap  sheets  are  sur- 
face volcanic  flows,  but  recognized  that  the.v  are.  in  part  at 
least,  later  in  age  than  the  bulk  of  the  Nipigon  rocks,  from 
the  fact  that  the  sheets  in  jilaces  cap  the  Nipigon  strataf. 

Irving  objects  to  the  idea  of  any  single  ••crowning  overflow," 
but  clearly  regarded  the  greater  i)art  of  the  trap  sheets  as 
contemporaneous  volcanic  sheets,  as  may  be  gather(*d  from  the 
following  (juotaiions: 

"So  far.  then,  as  1  have  been  able  to  learn  from  original 
observations,  and  by  reading  in  the  light  of  the  observations 
the  accounts  of  others,  the  Animikie  rocks  of  the  Pigeon  river- 
Thunder  bay  region  consist  of  a  great  series,  probably 
upwards  of  10.000  feet  in  thickness,  of  quartzites.  which  are 
often  arenaceous,  (puirtz  slates,  argillaceous  or  clay  slates, 
magnetitic  quartzites  and  sand-stones,  thin  limestone  beds,  and 
beds  of  a  cherty  and  jaspei-y  material.  With  these  are  associ- 
ated in  great  volume,  and  in  both  imbedded  and  intersecting 
masses,  several  types  of  coarse  gabbro  and  tine  grained 
diabase,  all  of  the  types  being  well  known  in  the  Keweenian 
series.!    Again  referring  to  Bell's  stratigraphical  scheme  for 


the      Animikie.      Irving    says: 


'Then 


agam. 


the     great 


♦Report  of  I'rosri'S^^,  drol.  Survt^.y  of  Ciiiisulu  184(>-?  p.  i:i. 
tGeol.  Survey  of  Oaiiiulii  Heporl  of  I'lOtrress  IWiB-tiO. 
Wopper-beiirltig  Rdcks,  i)..TOt. 


26 


BUI.LKTJN    NO.    ^'IIT. 


I  f 


■( 


volume  of:  included  beds  of  gabbro  and  diabas*'  is  almost 
entirely  ignored.  In  the  third  division  of  his  ticheme  it  is  said 
that  trap  beds  are  associated  witli  tliese  rocks  along  the  north 
shore  of  Thunder  bay,  at  the  Thunder  bay  mine,  and  in  the 
township  of  Mclniyre,  and  yet  the  whole  volume  of  this  divis- 
ion is  placed  at  only  4r)0  feet.  But  as  seen,  all  the  wny  from 
Wau&waugoning  bay,  on  the  Minnesota  coast,  to  the  south  side 
of  the  Kaminisfciquia  valley,  and  again  in  the  E'igeon  river 
country  of  Minnesota,  these  included  l^eds  must  aggregate 
over  a  thousand  I'eet  (?)  *while  they  may  be  nuch  more  than 
this  (V  ?).f  This  importavit  omission  is  pi'obably  to  be  ex- 
})lained  by  Bell's  having  regarded  all  of  these  beds  as  part  of 
the  so-called  "crowning  overflow"  which  is  supposed  to  have 
taken  place  after  the  accumulation  and  i-emoval  of  thousands 
of  leet  of  the  newer  Keweenian  or  copper-bearing  strata. 

"The  only  evidence  of  any  such  general  overllow  consists 
in  the  similarity  of  the  crystalline  rocks  found  capping  the 
hills  in  diiferent  parts  of  the  region.  Not  only  is  it  much  more 
in  accordance  with  the  geology  of  the  lake  Superior  region  to 
suppose  these  occurrences  to  represent  many  different  flows, 
but  there  is  distinct  evidence  that  they  do  so  in  many  cases. 
This  evidence  consists  in  ])art  in  actual  visible  inters tratiflca 
tion  with  the  slates  in  some  places  of  great  beds  o'"  olivinitic 
gabbro  identical  both  macroscopically  and  miscroscopically  with 
the  rock  capping  Tliunder  cape.  i\.noUier  evide.ice  is  the  very 
great  irregularity  of  level  of  this  .supposed  How  must  occupy, 
vhe  liight  at  which  it  is  found  varyin.'.r  back  and  forth  through 
distances  of  several  hundred  feet.  Yet  stronger  evidence  is 
found  in  the  general  structural  character  of  the  region,  by 
virtue  of  which  each  lieavy,  enduring,  ciystalline  rock  layer 
constitutes  a  ridge  with  a  long  front  slope  and  a  preclptious 
back  slope  "j. 

Selwyn,  in  18H4,  adopted  the  same  prevalent  views  as  to  *he 
partial  volcanic  composition  of  the  Animikie  group.  Writing 
in  that  year,  he  says:  "Between  Thunder  bay  and  the  east 
end  of  Nipigon  the  three  series  (Animikie,  Nipigon  and  Ke- 
weenian) follow  each  other  without  ap] parent  unconformity 
dud  dip  at  generally  low  angles  towards  t'le  lake.  Up  to  the 
summit  of  the  Nipigon  series  there  are  many  larger  inter- 
stradfied  beds  of  columnar  diabase,  then  follows  the  Keweenian 


'The  ((iicries  ari>  ilii'  Miitei's. 
*T!ic  (nierU's  lire  tlie  writer's. 
♦Op.  CIt.  pp   ;)HI    382. 


AI'COLITIC    SlI.Lh. 


27 


series,  consisting',  ote 


*  *  *  The  absence  of  paheoaLoiugicai 
evidence  of  a^re  may  be,  )ierhai).s.  in  a  ,<,'i'eat  measure,  accounted 
foi-  by  the  great  and  repeated  manifestation  of  volcanic  activity 
over  the  whole  rejjfion  durin<^  the  accumulation  of  the  sedi- 
ments, producing  conditions  higjily  unfavorable  foi-  the  exist- 
ence of  animal  life."'-  From  various  references  scattered 
throu.i^h  th(!  vvritin<,'s  of  Murray,  McFarlane.  Hunt  and  Mc- 
Kellar  it  is  clear  tViat  all  of  these  observers  regarded  the  sheets 
associated  with  the  xNniraikie  slates,  sandstone,  etc..  as  con 
temporaneous  volcanic  flows. 

The  late  Prof.  Alex.  Winchell  in  speaking  of  tlie  trap  sheets 
which  cap  the  Auimikie  slates  in  the  vicinity  of  the  interna- 
tional boundary,  used  th(?  following  v/ords:  "The  great  gab- 
bro  Hood.  1  agree  fully  with  American  geologists  in  assign- 
ing a  primitive  molten  condition  to  the  sheet  of  gabbro  which 
covers  so  many  hundreds  of  square  miles  it;  the  northwest. 
Bat  its  wide  extent  considered  as  a  molten  flood,  is  a  f a<?,t  which 
excites  amazement.  ••  *  *  *  *  a  total  of  thirty  nine  townships 
or  1,1  "/i' miles,  once  in  the  history  of  the  state  covered  by  a 
glowing  flood  of  molten  rock.f 

The  views  of  N.  H.  and  H,  V.  Winchell  are  ■^xprnssed  re- 
cently in  the  following  terms:  -The  Taconi(;  (Animikie)  in 
northeastern  Minnesota,  therefore,  with  some  period.-^  of  quiet. 
was  deposited  in  the  \uidst  of  violent  volcanic  disturbances  ?nd 
oceanic  transportation. '■; 

The  best  account  of  the  Animikie  grout)  that  has  yet  ap- 
peared is  that  given  by  E.  D.  Ingal!  in  his  report  on  Mines  and 
Mining  on  Lake  Superior.  || 

To  Mr.  Tngall  belongs  rhn  credit  of  taking  the  first  steps  in 
rectifying  cur'*ent  misconceptions  regarding  the  ge<)logi<'.al  re 
lations  of  the  trap  sheets  associated  with  the  Animikie  strata 
In  several  apecitic  instances  he  shows  in  this  pnper  that  the 
trap  sheets  are  not  conLempori.neous  Hows,  but  are  true  in- 
trusive sheets.  F^ut  notwithstanding  the  clearness  of  the  truth 
which  he  discovei'ed.  lie  .seems  to  have  been  so  inti.ienced  by 
the  weight  of  authority  of  ])revious  writei's  that  his  discovery 
remained  only  a  partial  one,  audits  full  significance  was  not  re- 
ali?;ed;  for  he  speaks  in  his  introduction  of  '-Sedimentary  and 
volcanic  rocks  of  the  Animikie. ""5?  and  a  litth)  further  on  ho  says. 


•Ooscripdvr  Skricli  of  ilic  IMiysica)  i  a Dijriiiili.V  iiiul  (a'u|i):;y  ul  (hf  l)oiiiiiiii)ii  of  Ciiii- 
sicla.  18S».  pii  :;i.-.'-J. 

'■  C:')!!^'*''^!  i""l  Nil  II  fill  liNI'iiy  survey  of  MliiiK'-nlii.  Mlxtci'iilli  iiiiiucit  ruport.  p.:)i51. 

*Irotioivs  of  Mli!ni's<>l!i,  Hull.  No.  V;.(}ool.  aiul  Niit.  Hist.  Survey  of  Miiiiio.sotu. 
l«n,p.  114, 

IIGeol.  ."Purvey  of  (JaniitlH,  tifnimil  iv.jon  18SH,  part  )I. 

SOp   I'lt.  p,S 


28 


BULLETIN    NO.    VI I L 


"the  rocks  comprising  this  silver  bearing  formation  (Aniiuikie) 
consist  of  basic  traps,  black  and  gray  argillites,  cherts  and  jas- 
pers, with  some  ferruginous  dolomyt,es  etc."*  The  evidence  of 
the  intrusive  character  of  some  of  the  :-heets,  which  Mr.  Ingall 
adduces,  will  be  reverted  to  in  the  sequel. 

Ic  thus  api)ears  from  a  review  of  these  references  to  the 
writings  of  most  of  the  geologists  who  have  had  occasion  to 
to  become  personally  familiar  with  the  Animikie  group  that 
the  uniform  teaching  has  been  that  the  series  is  partially  com- 
posed of  volcanic  rocks.  Most  observers  have  r<'i.''i'<loa  all 
of  the  igneous  rocks  associated  with  the  Animikie  series  as 
volcanic  surface  Hows.  Two  ob.servers  only,  In'ing  and  Ingall. 
have  recognized  the  intrusive  character  of  some  of  the  sh«^ets. 
and  of  these  Ingall  observed  so  many  cases,  that  it  is  a  matter 
of  surprise  that  he  did  not  attempt  to  generalize  .somewhat 
from  his  evidence. 

DLsnent  from  Jormer  vieic-s—This  doctrine  of  the  volcfidit. 
character  of  the  igneous  rocks  associated  with  the  Animikio  [':■> 
one  which  the  writer  is  disposed  to  deny.  The  writer  believes 
that  he  has  a  field  knowledge  of  the  Animikie  rocks  not  less 
extensive  .than  that  of  any  other  geologist  who  has  heretofore 
written  upon  then.  He  Icnows  them  fron^  Guntlint  lake  to  the 
mott  easterly  islands  of  Nipigon  bay.  He  has  examined  their 
entire  exposure,  in  all  its  magnificence,  along  the  coast  of  lake 
Superior  from  Grand  Portage  bay  to  its  most  easterly  occur- 
rence on  the  main  shore.  Traveling  on  foot,  he  has  examined 
all  the  sections  which  are  revealed  on  the  line  of  the  Canadian 
Pacific  railway,  both  east  and  west  of  Port  Arthur.  He  has 
followed  the  line  of  contact  of  the  Animikie  against  the 
Archaean  probably  more  closely  than  any  other  geologist  who 
has  put  his  observations  on  record.  He  has  visited  and 
descended  most  of  the  important  silver  mines  that  have  been 
in  operation  during  the  last  six  years.  He  has  examined  the 
well  knowji  exposures  on  the  wagon  and  canoe  route  from  Port 
Arthur  westward  by  wa\^  of  Rabbit  mountain,  the  Palisades, 
Silvei  Mountain,  Whitefish  and  Arrow  lakes,  and  the  lakes  of 
the  international  boundary.  He  has  also  traversed  the  Kam- 
iniptiquia  river  in  canoe,  and  has  made  many  minor  excursions 
throughout  the  district  occupied  by  the  Animikie  rocks.t 

♦  1(1  p.  23. 

■''riic  VMi'louH  evaiuliiations  wiiit'li  tlit'  wriler  liiis  timrte  fioin  I  iini'  to  liino  of  tli<'  Atii- 
iiilUic  loi'ks  liii  ve  1)1  L'ti  iiiciili'nta'.  to,  oi'  iipait  from,  his  niii.ioi'  work  Jii  Uk;  In  Wo  Super 
ior  rcjiion:  and  liis  kiiowlt'dfii- of  tlio  Hcrii's  lias,  tlit-rt'fore  hceii  Kalhfrcd  pi»'ci;-iiu>al. 
and  ha- li'd  to  no  sysiciuatlc  ai'i'otinl  of  iis  sjcolojrlt^'al  fi'alurcs.  Tlic  last  iioi^ossary 
and  iniporlani  inforniaiiori  was  olilalni'd  whUc  ixatnlnini?  the  I'oast  of  the  lal<i'  hi  the 
suninier  of  IS'.H.  Ill  (Icr  theaiispices  of  the  (ii^oloifical  and  Natural  lIlHlory  Hurvey  of 
Minnesota.  The  earlier  notes  wereohlained  while  eunductltiK  KOoloicai  gwofk  for  the 
(ieolosrical  Survey  of  Canada. 


LACCOLITIC   SILLS. 


29 


somewhat-  extensive 
the  writer  feels  war- 


With  these  exceptionally  favorable  opportunities  for  obser- 
vation, with  a  strong  interest  in  tlie  iuijeous  roelfs  of  the  region, 
and  having  always  had  in  mind  the  (luestion  which  is  to  l)e 
dealt  with  in  this  paper,  the  writer  has  failed,  after  a  prolonged 
.  and  critical  inquiry,  to  find  anywhere  in  the  Aniinikie  group 
any  trace  whatever  of  volcani<'  rocks. 

Views  here  wlvumrd — In  view  of  this 
familiarity  with  the  Animiltie  formations 
ranted,  from  his  failure  to  find  volcanic  i-ocks,  in  laying  down 
the  proposition,  that  there  arc  nn  amtetii/iDranmi/s  rolcanic  rocks 
i)i  the  Animike  group. 

He  dissents,  moreover,  from  the  vi(;w  which  has  been  enter- 
tained by  some  writers.  Bell,  particularly,  tliat  the  "trap  cap" 
or  so  called  "crowning  overtlow"  which  is  found  reposing  on 
the  Animikie.  is  a  volcanic  extravasation,  though  not  contem 
])oraL»'Ous,  since  the  same  sheet  has  been  observed  to  cap  also 
the  Nipigon  series;  and  he  maintains  that  iione  of  the  trap  sheets 
associated  witli  the  Animikie,  ivhe.fher  of  the  iiatun:  of  "ra/)^'"  or  in- 
tercalated sheets,  is  a.  roLcniic  flow.  He  further  maintains  that 
these  traj)  sheets  are  all  in'msive  in  their  origin,  and  are  of  the 
nature  of  laccolitic  sills. 

The  last  two  of  these  propositions  the  writer  believes  to  be 
susceptible  of  deiuonstration.  The  first  on  account  of  its  nega- 
tive character  cannot  be  so  surely  established.  But  since  these 
trap  sheets  are  the  only  rocks  vhich  liave  ever  been  sup])osed 
to  be  volcanic,  if  the  last  two  propositions  can  be  maintained, 
geologists  cannot  go  on  assuming  and  teaching  as  they  have 
done  in  the  past,  that  the  Animikie  group  is  partially  composed 
of  volcanic  rocks.  For  nowhere  in  geological  literature  is 
there  any  g<')d  evidence  recorded  of  the  existence  of  contem- 
poraneous volcanic  rocks,  and  in  none  of  the  hundreds  of  un- 
rivaled sections,  whi(  h  the  cliffs  and  mesa  scarps  of  the  Ani- 
mikie present,  has  any  igneous  root,  intercalated  with  the  sedi- 
mentary strata,  been  fo  „  i,  other  than  ^jr-.-g  trap  sheets.  An  at- 
tempt will,  tiierefore,  be  made  to  show  ,hu,t  these  trap  sheets 
are  intrusive  sills;  but  in  doing  this  no  effort  will  be  made 
to  treat  the  subject  exhaustively.  Tha  petrographical  char- 
acters of  the  rocks  will  not  be  entered  into  in  detail,  although 
they  present  a  very  inviting  lieid  for  investigation  from  certain 
points  of  view.  Neilhei  will  any  effort  be  made  to  array  great 
numbers  of  specific,  detailed  observations  of  field  relations; 
for  that  would  be  wearisome,  and  is  unnecessary  till  such  time 
as   the   views   here  })resented  be  challenged  and  their  truth 


80 


BULLETIN    NO.    VIH, 


doubted.  Tlte  general  statf^ments  which  the  writer  will  make, 
together  with  the  occasional  .s^)pcitic  references,  are  sutficient 
to  establish  his  thesis. 

PETliOOKAI'HICAL    CHAKACTKU   OF  THE  TRAP  SHEETS. 

The  so-called  "trap"  sheets  associated  with  the  Aniraikie 
group  embrace  two  distinct  types  of  rocks:  (1)  Diabase  (2; 
Granophyric  (|uartz-porphyry.  The  first  of  these  is  by  far  the 
most  abundant.  It  very  frequently  has  olivine  present  as  an 
im])ortant  constituent,  and  would  then  be  termed  an  olivine-di- 
abase.  In  structure  it  passes  locally  into  a  coarse  gabbro  or 
olivine-gabbro  on  the  one  hand,  and  on  the  other  into  a  fine 
grained  porphyryte.  The  prevailing  color  is  a  pepper  and  salt 
gray,  the  fresher  the  rock  the  lighter  being  the  color.  Where 
the  pyproxene  is  found  to  have  been  altered  to  hornblende,  and 
other  changes  have  taken  place,  the  rock  is  usually  darker  in 
color.  Opaque  iron  ores  are  generally  never  Ji'.bseni  from  the 
slidt  s  of  these  rocks.  The  ophitic  or  diabase  .structure  is  very 
prouounced  and  is  much  more  prevalent  than  the  ailotriomor- 
phic  granular  structure,  notwithstanding  the  fact  that  Irving 
and  others  commonly  refer  to  the  rock  as  gabbro.  An  abund- 
ant cro]!  of  phenocrysts  seems  to  have  been  developed  in  some 
few  localities  in  the  coarser  varieties  of  the  rock,  and  the  form- 
ation then  is  strongly  porphyritic,  as  on  the  shore  west  of  Cald- 
well point.  Another  feature  of  interest  but  also  quite  local  in 
its  occu'  rence  is  the  presence  of  irregular  patches  or  blotches 
or  blebs  of  quartz  in  the  mass  of  the  rock.  Thesfj  may  be  seen 
to  advantage  on  the  Canadian  Pacific  railway  abc  at  five  or  six 
miles  east  of  Port  Arthur.  Locally,  also,  the  cons'^ituent  feld- 
spar of  the  diabase  assumes  a  brick  red  color  and  the  rock  in 
consequence  has  a  reddish  color.  This  is  common  in  those  por- 
tions of  the  formation  which  are  charged  with  inclusu.ris  of 
quartzyte  fragments  from  the  Animikie  series,  but  is  also  ob- 
servable when  such  inclusions  are  not  to  be  found. 

This  assumption  locally  of  a  reddish  color  renders  it  some- 
times difficult  to  distinguish  the  diabase,  without  very  critical 
exarniuation,  from  the  second  petro»raphical  type  affected  by 
these  sheets,  viz..  the  (luartz-porphyry.  which  is  also  of  a  brick 
red  color,  and  frequently  has  a  ferro-magnesiau  silicate  as  a 
normal  constituent.  This  red  granophyrio  porphyry  is,  however, 
not  of  general  distributiou  like  the  diabase,  but  is  found  chiefly 
on  the  islands  .southwest  of  Pie  isiaud.  and  on  points  opposite 
Victoria  island  and  thence  southwestward  ou  the  mainland.  In 
some  of  its  phases  it  resembles  some  of  the  various     r-»d-rock" 


Hiwi^UvWiHwiyiw'/ . 


LACCOLITIC   SILLS. 


31 


formations  on  the  Minuosota  coast,  which  have  by  former  inves- 
tigators been  chissed  as  Keweenian  tlovvs.  Most  of  it  seems  to 
be  indistinguishable  in  its  essential  characters  from  the  "red 
r  )ck"  of  Pigeon  point  which  has  been  so  ably  investigated  by 
Prof.  Bayley,  who  indentities  it  as  soda  granite  and  quartz-kera- 
tophyre.  and  advances  a  remarkable  explanation  of  its  mode  of 
development,  which  the  writer,  after  a  careful  examination  of 
the  conditions  revealed  on  Pigion  point,  believes  to  be  correct.* 
The  area  occupied  by  these  sheets  of  red  rock  within  the  Ani- 
mikie  province  is,  however,  small,  and  some  portions  of  them 
vary  considerably  from  the  true  quartz- porphyry  fades,  and  ap- 
pear from  their  held  relations  to  be  intermediate  graduations 
between  an  acid  <|uartz-porphyry  and  the  diabase,  as  if  by  the 
mixture  of  acid  and  basic  magmas.  To  establish  such  a  sup- 
position would,  however,  require  a  large  amount  of  thorough 
petrographical  study.  Prof.  Bayley's  careful  work  at  Pigeon 
point  sets  the  example  of  the  true  method  of  attacking  these 
highly  interesting  problems  in  petrogeny.  There  is  no  place 
for  their  discussion  in  this  paper. 

CoriNtancy  of  diarader — The  sheets  of  dia,base  are.  with  the 
local  exceptions  above  mentioned,  remarkably  constant  in  their 
petrographical  characters  over  wide  areas  of  country.  They 
are,  for  example,  invariably  holocrystalline  and  no  glassy  por 
tions  have  anywhere  been  detected  in  them.  They  are  nevei- 
amygdaloidal,  a  fat-t  wiiich  has  been  frequently  noted  by  earlier 
writers.  In  this  respect  the  trap  sh<^ets  present  a  very  marked 
contrast  to  the  volcanic  lavas  of  the  Keweenian,  which  are  gener- 
ally amygdaloidal.  This  total  absence  of  amygdaloidal  and 
vesicular  structure  is  a  very  strong  argument  in  itself  against 
these  sheets  being  surface  tlows.+  The  sheets  also  have  very 
constantly  a  columnar  or  sub-columnar  structure  transverse  to 
the  plane  of  their  extension.  This  columnar  structure  moreover 
extends  through  the  entire  sheet,  from  the  lower  surface  to  the 
upper.  This,  again,  is  in  i-ontrast  wiMi  the  columnav  structure 
observaole  in  thick  surface  Hows.  In  these  cases  the  columns 
are  also  prevalingly  at  right  angles  to  the  cooling  surfaces,  but 
they  do  not,  so  far  as  the  observations  of  the  writer  serve  him. 
extend  comi)letely  through   from  surface  to  surface.     In  their 


*Ofi"ln()t  thoSoiilii  Ciruiiitf  iind  i,>uiirl/.-k(^niliipli>  ivof  PiL't'cm  point,  by  \V.  S.  Bay- 
k\V  Am..I()ur.  of  Sci..  Vol.  XXMX  April.  US'.)(). 

till  ciiie  insrsiuoe  dht'  w  riu-r  lias  observod  an  aiuyjidiiloidul  rock  in  the  Aninilkie  prov- 
inoo,  but  this  Inis  no  connection  with  tlie  trap  sht-ets  and  is  probably  an  out-lylnjc 
I'eninant  of  tin'  Ki^wei-nlan.  I'f.  Note  oii  tin-  oiuMirreuco  of  native  copper  in  the 
AuitniUi.-  locks  nf  Tlmnder  hay.    The  American  Geoloffist.  March,  ISOl). 


82 


BULLETIN    NO.    S'lll. 


columnar  structure  these  trap  sheets  are  entirely  analogous  to 
the  numerous  dj'kes  of  the  region,  in  which  the  columnar 
structure  is  pronounced  from  wall  to  wall  of  the  dyke. 

Petrographical  differentiation  —While  thus  emphasizing  the  con- 
stancy of  the  petrographical  character  of  these  sheets,  it  is  not 
the  intention  to  assert  that  there  is  no  yjetrographical  differen- 
tiation within  the  sheet  itself.  Theie  is.  indeed,  a  very  pro- 
nounced and  evident  variation  in  the  character  of  the  rock 
comxjosing  each  sheet.  But  this  variatioi;  obeys  a  definite  law. 
and  Is  constant  for  every  sheet,  so  that  the  variation  itself  be- 
comes an  invariable  character,  wherever  the  full  thickness  of 
the  sheets  may  be  observed.  This  petrographical  variation  is 
strictly  analogous  in  its  physical  aspects  to  the  dilTerentiation 
which  the  writer  has  described  as  characteristic  of  the  dykes 
which  traverse  the  Archaean  terranes  in  the  Rainy  lake  region.* 

Whether  there  is  also  a  corresponding  chemical  variation  as 
in  the  case  of  the  dykes,  has  not  yet  been  investigated.  Where- 
ever  the  contact  of  the  trap  sheets  with  the  Animikie  slates 
has  been  observed,  whether  at  the  lower  or  the  upper  surface 
of  the  sheets,  the  diabase  at  the  immediate  contact  has  been 
found  to  be  a  dense  compact  rock,  either  microcrystalline  or 
quite  aphanitic  with  occasional  minute  phenocrysts  of  plagio- 
clase  scattered  through  it.  The  texture  of  the  rock  becomes 
coarser  rapidly  as  the  distance  from  the  contact  is  increased, 
and  if  the  sheet  is  thick  the  rock  is  usually  quite  coarse  at  the 
distance  of  a  few  yards  from  the  contact.  In  the  structure,  as 
exhibited  in  a  number  of  representative  slides  from  different 
localities,  the  rock  varies  from  that  of  a  dense,  very  fine  grained 
diabase-porhyryte  into  that  of  a  fine  grained  diabase,  the  ophitic 
structure  being  pronounced,  and  then  to  that  of  a  coarse  diabase 
with  a  tendency  in  places  to  assume  a  granular  structure. 

Analogous,  though  somewhat  different,  variations  in  texture 
and  structure,  also  obtain  for  the  sheets  of  red  granophyric 
quartz  porphyry,  at  least  so  far  as  their  lower  contact  is  con- 
cerned, as  may  be  clearly  observed  on  Victoria  island.  These 
variations  in  texture  and  structure,  particularly  in  the  diabase, 
are  well  known  to  be  characteristic  of  the  contact  phenomena 
of  intrusive  masses;  and  taken  together  wit^  the  strong  simi- 
larity in  the  field  aspect  of  the  sheets  tc  the  dykes  of  the 
region,  the  presuL^ption  in  favor  of  regarding  them  as  hori- 
zontal dykes  or  intrusive  sills  becomes  very  strong. 


♦r'lilrographical  niffeiiMitiiiiioii  of  Cortaiii  Dykos  of  the  Kainy  Lake  Region. 
Amcrivan  Geologist,  March,  1891. 


Tht; 


LACCOLITJC    SILLS. 


;3n 


SOME  MH(JAI)  FEATURES  OF  THE  TRAP  SHEETS. 

Their  Slnii'lcUi/- -Another  argument  in  favor  of  the  intrusive 
character  of  the  sheets  is  their  simple  and  uniform  character 
regarded  as  geological  masses.  In  this  they  again  resemble 
the  great  dykes  that  traverse  the  region.  In  the  case  of  sur- 
face flows,  whether  from  volcanic  craters  or  from  fissure  out- 
wellings,  it  is  an  exceedingly  abnormal  condition  of  alfuirs  to 
find  the  extravasation  take  the  form  of  a  sinr/lc  sheet,  occupy- 
ing hundreds  of  sfiuare  miles  and  of  practically  uniform  thick 
ness.  Yet  tliis  is  precisely  the  character  of  the  sheets  under 
consideration.  Their  simple  individuality,  their  regularity, 
the  total  absence  of  over  lappings  of  one  sheet  oh  another,  are 
features  which  are  sufficient  to  negative  the  supposition  that 
they  are  surface  tlows.  The  unity  and  persistence  of  the 
sheets  over  wide  areas  is  remarkable.  The  Animikie  strata 
and  these  associated  trap  sheets  have  together  been  dislocated 
by  a  great  system  of  faults;  and  the  orographic  blocks  so 
formed  have  been  vei'y  frequently  tilted,  so  as  to  present  a  long 
gentle  slope  to  the  south-east,  and  a  steej:)  scai'p  to  the  north- 
west The  angle  of  tilt  has  been  small  ranging  usually  from 
0°  to  5°,  but  is  sometimes  higher;  and  portions  of  the  region 
afford  a  remarkable  illustration  of  this  tilted  structure,  and  of 
a  topography  conditioned  by  it.  The  many  long,  narrow  lakes 
which  occupy  the  Animikie  province  in  the  vicinity  of  the 
international  boundary  clearly  lie  in  fault  lines,  between  a  steep 
scarp  on  one  side,  and  a  more  or  less  gentle  slope  on  the  other. 
The  non-recognition  of  this  prevalent  tilted  structure  has  been 
the  origin  of  much  confusion  in  the  descriptions  of  the  Ani- 
mikie group;  and  very  excessive  estimates  of  the  thickness  of 
the  series  have  been  made  both  by  Irving*  and  by  Ingall.t 

In  the  opinion  of  the  writer  one  fifth  of  these  estimates  is 
much  nearer  the  true  thickness  than  the  figures  given  by  these 
geologists.  The  recognition  of  the  tilted  structure  renders 
the  correlation  of  discrete  portions  of  a  single  sheet  a  matter  of 
no  great  difficulty  to  a  stratigraphic  geologist.  Such  a  corre- 
lation, based  upon  correct  ideas  of  the  structure,  shows  cleaiiy 
that  the  trap  sheets  are  few  in  number,  and  single  sheets  may 
be  traced  in  geological  continuity  for  many  hundreds  of  square 
miles.  This  persistence  of  single  sheets,  more  noticeable  in  the 
thick  "trap  caps"  than  in  the  thinner  intercalated  sheets,  is  even 


♦lO.O'O  fw!t.    Coppt'i'-heiirin?  Uocks  p.  HSO. 

tl2,iKH)  feet,    liepcirt  on  Mines  and  Minius  la  Lake  Superior  p.  ^6. 


84 


HULLKTIN    NO.    VIII. 


more  evident  on  the  lake  front,  whore  tilting  is  le.ss  pronounced 
and  the  sheets  may  be  traced  in  actual,  as  well  as  geological 
continuity,  over  equally  great  areas  with  practically  uniform 
thickness.  In  other  cases,  where  the  level-topi)ed  mesas  are 
dissected  by  erosion  and  separated  into  several  topographical 
masses  by  valleys  of  greater  or  loss  width,  there  can  be  no 
reasonable  doubt  of  the  original  continuity  of  the  separate  caps 
of  the  now  isolated  hills,  since  they  occupy  the  same  levels, 
overlie  the  same  rocks,  have  the  same  general  thickness,  and 
the  same  peculiarities  of  petrographical  detail  both  macrosco- 
pic and  microscopic.  This  persistence  and  the  practical 
uniformity  of  level  (prior  to  tilting),  together  with  the  fairly 
uniform  thickness  of  the  sheets  constitute  a  combination  of 
features  not  affected  by  surface  flows. 

Absence  of  Pijrocla-stic  Rocks — The  sheets  are  nowhere  asso- 
ciated with  pyroclastic  rocks  such  as  would  suggest  their  ex- 
travasion  at  the  surface.     There  is  no  ash,  or   tulf,    or  coai'se 
agglomerate.     Neither   is  there  any  trace  of  breccia  or  rece- 
mented  lava  fragments  of  any  kind,  such  as  are  commonly  de 
veloped  in  the  progress  of  a  surface  flow  of  lava. 

Absence  ofjiow  slrucfiire — There  is,  moreover,  a  complete  ab- 
sence of  flow  structure  such  as  is  so  abundant  in  the  lavas  of 
the  Keweeniar  This  is  true  of  the  surface  of  the  sheets  as 
well  as  of  their  internal  parts.  The  surface  of  the  Keweenian 
lavas  is  very  frequently  characterized  by  the  presence  of  wrink- 
les and  of  other  manifestations  of  what  is  known  as  "ropy 
structure."  These  features  are  never  found  on  the  surfaces 
of  the  trap  sheets,  even  where  these  are  freshly  exposed  by 
the  recent  removal  of  the  over- lying  slates. 

The  enclosing  rocks — Another  feature  which  does  not  accord 
with  the  supposed  surface  character  of  these  sheets  is,  that  the 
sedimentary  strata  below  the  sheet  are  usually,  so  far  as  ob- 
sei'vation  is  possible,  the  same  as  the  strata  which  immediately 
overlie  it.  Of  course,  as  regards  the  thick  trap  caps  of  the 
region,  there  are  generally  not  now  any  strata  reposing  upon 
them,  and  it  is,  therefore,  impossible  to  say  what  the  nature  of 
the  overlying  beds  originally  was.  But  as  regards  the  inter- 
calated sheets,  the  statement  is  true  that  there  is  no  essential 
difference  in  the  character  of  the  enclosing  rocks  above  and 
below  the  sheet;  and  on  the  surface  of  the  thick  trap  sheet, 
which  extends  inland  from  the  town  of  Port  Arthur  and  skirts 
the  north  shore  of  Thunder  bay,  and  which  is,  for  the  most 
part  a  cap  sheet,  there  are  numerous  remnants  of  the  overlying 


f.ACCOLITIC    SILLS. 


35 


Aniraikie  slates  still  adhorini,'  to  the  trap  (soo  Plate  VT.  Fronfifi- 
pivcr)  Vv'hich  aro  the  same  rode  essentially  as  t.  e  slates  which 
uiulerlie  the  sheet. 

Intersertion  of  stnihi  Inj  thr  s//rr^v— Ovvinj?  to  the  little  disturbed 
couditioii  of  the  Animilcie  sti-ata.  and  to  the  fact  that  the  sheets 
had  their  origin  prior  to  the  disturbance,  the  sheets  appear 
(•(jmmonly  to  follow  in  each  ease  a  sin<rle  geolo2:ical  horizon. 
To  this  appar<!nt  rule,  however,  there  are  very  numerous  ex- 
ceptions, and  generally  if  the  sheets  are  folk)»ved  in  detail 
their  surfaces  are  found  vat  to  be  strictly  parallel  to  the  bed- 
ding ])lanes  of  the  Animilcie  strata,  but  Xo  intersect  them  t;0  a 
greater  or  less  extent.  (Generally  the  intersection  is  of  a  minor 
character.  Sometimes,  however,  it  amounts  to  an  im]>ortant 
])assuge  from  one  geological  horizon  to  a  very  ditlerent  one. 
Many  of  these  intersections  of  the  Animikie  strata  by  these 
trai>  sheets  have  been  observed  and  recorded  by  Ingall.* 

One  or  two  cases  have  been  noted  by  Irving,  t  and  others 
have  been  observed  by  the  writer.  One  of  these  intersections 
is  well  illustrated  by  Ingall  in  the  lower  figure  of  the  i)late 
facing  page  24  in  the  report  above  referred  to.  Although  this 
is  entitled  an  illustration  of  a  "Trap  flow  on  Argillytes;*'  it  is. 
undoubtedly,  an  intrusive  contact,  as  Mr.  Ingall  himself  is  dis- 
posed to  admit.  In  addition  to  the  observations  of  Ingall  and 
Irving  a  few  other  cases  noted  by  the  writer  on  the  coast  may 
here  be  given.' 

On  the  face  of  the  high  bluff  which  rises  on  the  south  side  of 
Sturgeon  bay,  the  traj)  cap  may  be  seen  not  only  to  abruptly 
change  its  thickness  and  to  pass  up  over  the  edges  of  the  slates, 
but  also  to  send  a  long  tongue- like  prolongation  within  the  lat- 
ter apparently  parallel  to  the  bedding.  On  the  south  side  of 
Prince's  bay,  in  passing  eastward,  the  trap  cap  is  seen  to  de- 
scend across  the  bedding  of  the  slates  to  the  water's  edge;  and 
and  at  the  point  which  forms  the  headland  of  the  bay  it  appears 
clearly  to  cut  the  slates.  Southward  from  the  point  the  trap 
continues  to  cut  the  slates  for  two  or  three  hundred  yards, 
and  then  passes  up  over  them  and  merges  with  the  trap  cap 
once  moi'e.  The  appearance  is  very  much  at  one  place  as  if 
there  were  a  large  dyke  in  direct  continuity  with  the  trap  cap. 
At  the  same  locality  is  another  dyke  distinctly  cutting  both 
slates  and  trap  cap. 


*Op.  Git.  pp.  42.  46,  Til,  80.  !).». 
+Op.  Cit.  pp.  873.374. 


86 


HULLIOTIN    NO.    VIII. 


On  tho  north  sid(>  of  Little  Trout  buy  ut  a  point  about  N.  N. 
E.  of  McKollar's  point,  may  be  seen  a  very  clear  case  of  an 
intrusive  sill.  Tin?  sill  is  al)out  four  feet  thick  and  is  composed 
of  a  dark  ^'ruy  diabas(>.  It  has  boon  inj(;ctod  alon<.,'  a  struc- 
tural plane  in  an  older  and  tliick(;r  sheet  of  a  pirdtish  weather- 
ing trap  which  presents  a  marked  contrast  to  the  sill.  The 
latter  has  a  dip  of  probably  10  to  the  southward.  It  is  at  the 
water's  ed<x«5  and  i)resents  a  perfo(;tly  clean  exposure.  The 
upper  and  lower  contacts  of  the  sill  with  the  enclosing,'  rock 
are  renuii-kably  sharp  and  the  details  of  this  contact  show  tho 
intrusive  character  of  the  sill.  The  petrographic  variation 
within  the  sill  \i^v\f.  establishes  the  same  fact.  It  is  a  dense 
aphanitic  rock  at  both  upper  and  lower  contacts,  and  in  pass- 
ing: inward  from  the  enclosing  walls  on  either  side  towards 
the  middle,  the  rock  rapidly  assumes  a  coarser  texture  and  in 
the  middle  is  a  medium  coarse  grained  ro(!k. 

On  tho  south  side  of  Little  Trout  bay,  about  its  middle,  the 
gray  trap  may  be  seen  mounting  up  over  the  abrupt  edges  of 
the  rtat  Aniuiikie  slatos  and  then  continuing  along  u])on  their 
upper  surface  as  a  cap.  The  relations  are  diagrammatically 
j-epresented  in  the  fig.  1.     Practically   the  same  relationship 


MMSSS^SMSi 


\'\iX.  1.     niasraiiuiiittk'  spctloii  showliij?  thf  rolation  of  the  tr!i|>  cii))  to  tlio  Aniinikie 
slates  on  the  south  side  of  Little  Trout  bay. 

of  trap  and  slate  may  bo  seen  on  the  south  side  of  the  extrem- 
ity of  McKellar's  point.  Here  again  the  trap  mounts  over  the 
edge  of  the  fiat  slates  and  then  continues  as  a  horizontal  sheet 
resting  upon  them.  There  is  here,  liowever,  one  interesting 
point  of  ditference  from  the  conditions  last  referred  to.  The 
edges  of  the  flat  slates  do  not  abut  squarely  upon  the  intersect- 
ing trap,  but  are  sharply  bent  and  broken  at  the  contact,  and  in 
this  appear  to  atTord  evidence  of  a  fault  strain  or  monocline 
flexure  which  preceded  the  rupture  of  the  slates.  (Pig.  2.) 
On  the  north  side  of  Big  Trout  bay  near  the  end  of  the  bay. 
the  great  vertical  clitTs  show,  besides  the  common  thick  cap  of 
gray  trap  on  the  slates,  tWo  very  well  marked  sills  at  lower 
levels  and  near  the  shore.    ^.    .ese  are  nearly  horizontal  and  lie 


i.Acicoi.rnc  sili.k. 


87 


parallol  willi  Iho  boddin^',  ilie  ui)i»or  ono  l)»'iii<^  Id  or  IL'  foet 
thick  and  the  lowor  out!  4  (;i'  "i  foet.  Thoy  aro  very  distinctly  col- 
\imtiar  and  very  well  dotincd  l)asaltii'  columns  of  siniill  dinion- 
siotis  may  be  easily  removed  from  the  ed^^'t's  of  thi'  she((ts. 
Both  of  these  sheets  are  tine  <jfrain(!d  aphunitie  rocks  at  the  top 
and  bottom,  and  assumt!  a  coarser  texture  towards  the  middle. 
The  details  of  the  contact  with  the  slates  on  both  the  upper 
and  lower  sides  of  sheets  establish  their  intrusive  character, 
there  beinj?  small  intersections  of  the  bedding'  i)lanes  of  the 
slates. 

Near  the  extremity  of  the  point  which  foi-ms  the  south  head- 
land of  Bij?  Trout  bay  the  trap  is  ajjain  seen  to  cut  iii)  tlirou^h 
the  llat  slates  and.  after  i-eaching  a  certain  level,  bo  bo  extended 
over  them  in  the  form  of  a  horizontal  slif^n  or  cap. 

On  the  south  side  of  this  same  pijirit,  also  near  the  extremity, 
the  relations  of  trap  and  slate  are  revealed  in  a  very  clear  and 
instructive  cliff  section.  The  trap  here  again  mounts  up  over 
the  abrupt  clean  edges  of  the  slate  and  is.  after  reaching  a 
certain  lev(»l,  extended  hori/^ontally  upon  them  in  the  form  of 
a  cap.  The  upper  surface  of  the  slate  formation  is  shari)ly 
step- like  in  places  and  uneven,  and  the  contact  of  the  trap  and 
slate  is  therefore  not  a  simple  i)lane.  but  an  irregular  surface 
of  fresh  rupture  along  neighboring  bedding  planes  with  abrupt 
descents  from  one  plane  to  anotlier.  Two  thirds  of  the  way 
down  the  cliff  is  a  second   trap  sheet,  very  much  thinner  and 


mm^mm^ 


F"ig.  2.     l)iiifiiiiiHiii:itic  scctinii  mi   the  siuitii  side  iif  the  I'Xti't'tnil y  of   McKoUiir'> 
point  showinsr  I  lie  icl.ilioii  of  the  triip  cup  tot  lie  AiiiiiiiUi<;  strjitii. 

intercalated  with  the  slates.  This  is  clearly  an  injected  sill,  for 
it  may  be  seen  in  the  cliff  face  to  pass  from  one  bedding  plane, 
across  the  edge  of  the  slates  above  and  below,  to  another  hori- 
zon about  10  feet  higher.  In  the  same  section  there  is  also  a 
vertical  dyke  which  seems  to  antedate  the  horizontal  sill.  The 
relations  observed  are  represented  diagrammatically  in  figure 
three.  The  relation  of  the  small  sill  to  the  thicker  mass  of 
trap  could  not  be  clearly  ascertained. 


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23  WEST  MAIN  STREET 

WEBSTER,  NY.  )4580 

(7U)  872-4503 


f^" 


g 


38 


BULLETIN    NO.    Vlll. 


An  intorestinf?  case  of  an  intrusive  sill  is  shown  in  the  cliff 
section  on  the  southeast  side  of  Victoria  island.  Hero  a  four- 
foot  sill  of  gray  diabase  has  been  injected  aloiijr  the  plane 
of  contact  between  the  Animikie   slates  and  an   earlier  .sheet 


—  '  '       _     J  ■ ^^■**^^?*«^ — ;■  ^^     i'l    t/(il       1/    IJ 


1  Lml  .iiititt 


sui 


FIr.  3.     DlrKtiiiiiiiiiitlcsi'clioii  near  the  exfi-enilty  of  llic  point  Ix'twccn  Hiu'  Trout  ati<l 
PIkood  bays,  showing  the  reliitlon  of  the  trap  sIHs  lo  the  Aniniikii'  Ntiata, 

of  red  quartz-porphyry.  The  sill  has  a  dip  of  10"  to  15  .  and 
has  the  usual  sharply-marked  gradations  in  texture  and  struc- 
ture in  i)assing  from  the  upper  and  lower  surfaces  towards  the 
middle  of  the  sheet.  The  difference  in  density  of  the  rocks 
suggests  that  this  has  been  the  cause  determining  the  con- 
tact plane  of  the  quartz-porphyry  as  the  plane  of  intrusion.* 
The  local  thickness  of  the  sheet  of  quartz-porphyry  is  about 
forty  or  fifty  feet. 

Passcif/e  of  ■sheets  to  the  Horhtui  of  the  KcweenUth — Rut  not  only 
are  there  numerous  instances,  such  as  those  just  cited,  where 
the  trap  sheets  cut  across  the  bedding  planes  of  the  Animikie 
slates,  but  on  following  the  coast  eastward  from  Port  Arthur 
it  becomes  apparent  that  the  same  sheets  which  cap  the 
Animikie  pass  over  to  a  hisrher  geological  horizon  and  cap  the 
local  stratigraphic  column  of  the  Keweenian  (Nipigon).  as  was 
clearly  recognized  by  Bel  If  many  years  ago. 

In  the  vicinity  of  Black  Sturgeon  river,  and  on  the  shores 
and  islands  of  Nipigon  bay.  there  are  abundant  illustrations  of 
great  cliffs  of  Keweenian  strata  capped  by  thick  sheets  of  trap, 
which  are  Jentical  with  those  which  cap  the  Animikie  series 
at  Thunde  •  bay;  and,  although  these  sheets  cannot  be  ti'aced  in 
absolute  conformity  in  the  interval,  there  are  many  outlying 
patches  which  fill  the  gap,  and  the  writer  agrees  with  Bell  in 
the  belief  that  there  is  no  reasonable  doubt  as  to  the  geological 
continuity  of  the  trap  sheets  of  Nipigon  and  Thunder  bays,  al- 

•On  this  pohit  i-f.  Gllhert,  (ieohijry  of  tlie  Mciiry  Mouiit.-iiiis 
top.  Clt. 


/;'.] 


»»ft. 


LACCOLITK!   SILLS 


;!•» 


y 


thouj^h  he  rejects  Bell's  view  tliat  thoy  represent  a  volcanic 
overflow.  These  sheets.  cappin<r  the  Keweeuiau  of  Nipij^on 
bay,  are.  of  course,  distinct  from  the  true  Kewoenian  lava  beds 
and  cannot  be  confounded  witli  them.  The  relations  of  the 
trap  sheets  to  tlie  Kewetmian  strata  are  the  same  as  those 
which  they  bear  to  the  Animikie  strata.  Not  only  do  the  sheets 
rest  as  caps  on  the  mesas  and  tilted  oi'ographic  blocks  of  the 
Keweenian.  but  they  also  cut  across  the  beds  oi  the  latter,  pre- 
(usely  as  they  do  in  the  Animikie.  One  of  the  best  instances 
of  th(»  evident  intrusion  of  tliese  g^reat  trap  sheets  is  near  the 
mouth  of  Nipigon  river,  at  the  great  red  rock  clitf  which  rises 
perpendicularly  alcove  the  Canadian  Pacitfc  railway  track. 
Here  a  great  mass  of  the  diabase  rises  from  the  level  of  the 
lake,  cuts  aci'oss  the  hoi'izontal  bedding  of  the  Keweenian  for 
a  vertical  distance  of  140  feet,  and  th(m  spreads  out  over  the 
surface  in  the  form  of  a  great  sheet  llT)  feet  thick.  The  Ke 
weenian  strata  here,  are  very  largely  made  up  of  bright  ver- 
milion red  sandstones  and  purplish  shales.  These  at  the  con- 
tact and  for  a  hundred  feet  or  so  from  it  have  been  much  dis- 
turbed, and  have  been  bleached  white.  Two  small  dykes,  a])- 
parently,  though  not  demonstrably,  apophyses  from  the  great 
er  mass  of  intrusive  rock  cut  the  sandstone  and  shales  near 
this  zone  of  alteration.  The  relations  exhibited  in  the  section 
are  repre.sented  diagrammically  in  flig.  4,  the  outlines  of  which 
are  taken  from  a  photograph. 


/:'»// 


'JLfvft  of  l^ake  Su/itfim 


I'Mr.  4.  IiitiM'sfctloii  of  Kfwi'iMi!;iii  ■slriiMi  by  the  tf.'ip  sheet  and  llif  oxti'ntloii  of  Iho 
liittor  tis  :i  thick  oup  (ivcr  ilic  liDii/.oiital  IiimIs.  Ued-ruck  i-lill'  at  the  iimuth  of  Nipi)roit 
river. 


In  this  region  the  Animikie  is  lacking  and  the  higher  Ke- 
weenian series  rests  directly  on  the  Archiean.  On  both  sides 
of  Mazokamah  bay.  also,  the  trap  sheets  may  be  seen  in  the 
face  of  the  the  high  bluffs  to  cut  across  the  ruptured  Kewoenian 
strata. 


40 


BULLETIN    NO.     VlJl. 


At  liossport,  at  the  extreme  east  end  of  Nipij^on  bay,  the 
Animikie  slates  aj^ain  ajjpear  between  the  old  Arcluean  base- 
ment and  the  Keweenian  strata,  and  hero  again  the  same  trap 
may  be  observed  as  a  single  mass  both  to  cut  the  Animikie 
slates  vertically  and  to  caj)  them  horizontally. 

THE  LOWER  CONTACT  OF  THE  TRAP  SHEETS. 

A  critical  study  of  the  details  of  the  contacts  of  the  trap 
sheets  with  the  enclosing  rocks  yields  sutticient  evidence  lo 
demonstrate  their  intrusive  character.  Of  course  the  fact  that 
the  great  bulk  of  these  sheets  takes  the  form  of  mesa  caps,  from 
which  the  overlying  slates  have  been  removed  almost  entirely, 
renders  it  difficult  to  multiply  observations  on  the  upper  con- 
tact. But  even  on  the  surface  of  some  of  the  ti-ap  caps  there 
are  sufficient  remnants  of  the  former  volume  of  overlying  strata, 
to  afford  precise  and  absolutely  decisive  evidence  of  their  intru- 
sive origin;  and  from  the  essential  Identity  of  the  character  of 
all  the  sheets,  it  would  be  a  fair  inference  to  conclude,  that  if 
one  trap  caj)  can  be  demonstrated  to  be  intrusive,  the  others  of 
the  region  of  precisely  the  same  j)hysical  features,  and  alllack- 
ing  the  character  of  volcanic  Hows,  are  also  intrusive.  But  the 
thesis  here  laid  down  does  not  depend  upon  such  an  inference 
for  its  establishment.  The  lower  contacts  of  the  trap  caps 
with  the  strata  upon  which  they  rest  may  be  studied  in  practi- 
cally an  endless  number 'of  clean,  bare,  rock  walls;  and  the 
facts  observed  along  the  lower  surface  of  the  sheets  is  of  high 
critical  value — sufficient  to  demonstrate  their  intrusive  charac- 
ter. On  the  supposition,  which  has  been  heretofore  enter- 
tained, of  the  volcanic  character  of  the  trap  sheets,  there  are 
two  possible  cases  to  be  considei*ed;  (1)  the  tlows  might  have 
been  extravasated  at  the  time  of  the  deposition  of  the  Animi- 
kie sediments  which  immediately  underlie  them.  (2)  They 
might  have  been  extravasated  after  the  induration  and  partial 
denudation  of  the  underlying  Animikie  rocks. 

In  the  first  of  these  two  suppositious  cases,  it  is  clear  that  a 
lava  flowing  over  soft  sediments  of  a  clayey  character,  such  as 
must  have  been  the  original  condition  of  the  slate  commonly 
found  beneath  the  trap  caps,  would  give  us  a  confused  and  very 
indefinite  plane  of  contact;  the  sediments  would  be  disturbed 
by  the  encroachment  upon  them  of  a  great  thick  lava  flow,  and 
there  would  be  more  or  less  of  a  mixture  of  lava  and  sedi- 
ments. All  this  is  very  far  from  being  the  condition  which  pre- 
vails at  the  contact  of  the  trap  sheets  with  the  rocks  upon  which 
they  repose.     That  contact  can  almost  invariably  be  located 


LACCOLITIC   SILLS. 


m 


with  knifo  ed^e  })recision,  (see  fig.  o).  There  is  never  any  con- 
fusion of  sedimentary  rocks  with  the  triii).  and  tlio  sharply 
angular  character  of  the  broken  edges  of  the  slates,  where  they 
are  imbedded  in  the  trap,  shows  clearly  that  the  slates  were  not 
in  a  soft  condition  at  the  time  they  were  covered  by  the  magma. 


11- 


Kij?.  11.    Conlai't  of   the  lowi'i-  stiifiici-  of   llie  liiip  <'H|>  willi  imdci  lyiiit;   Aiiliiilkir 
strata  on  tlii>  .shore  opposlto  Vii-toriu  Isluiul. 

The  filling  of  minute  fissures  and  cracks  in  the  slate  by 
apophyses  from  the  trap  sheet  shows  the  same  thing.  The 
slates,  then,  were  hard  and  brittle  and  had  acquired  their 
characteristic  cleavage  prior  to  the  advent  of  the  traps.  The 
numerous  cases  of  the  trap  sheets  cutting  across  the  bedding 
planes,  as  above  recited,  also  shows  the  ti'aps  to  be  later  than 
the  induration  of  tl>e  slates  and  the  development  of  the  cleav- 
age. It  is  clear,  tlion.  that  even  if  the  trap  caps  ai'e  volcanic 
Hows,  they  are  not  contemporaneous  with  the  deposition  of  the 
Animikie  sediments.  If  then,  they  are  volcanic  tlows  at  all  they 
come  under  the  second  of  the  two  possible  suppositious.  This 
implies  that  they  were  extravasated  as  flows  on  an  eroded  sur 
face  of  the  Animikie  rocks.  The  study  of  the  contact  lends  no 
countenance  to  this  supposition.  An  eroded  surface  has  two 
pronounced  features  which  are  absolutely  lacking  in  the  surface 
upon  which  the  trap  caps  repose.  These  are  (1)  the  common 
evidences  of  sub-aerial  weathering  and  the  sculpture  of  the  sur- 
face, and  (2)  the  accumulation  of  surface  debris.     The  absence 


42 


BULLETIN    NO      VIH. 


of  these  features  in  any  single  section,  or  even  in  several  locali- 
ties, would  not  be  conclusive;  but  when  it  is  remembered  that 
the  nature  of  the  exposures  is  such,  that  the  unobscured  con- 
tact may  be  critically  examined  along  hundreds  of  miles  of 
irregularly  winding  mesa  scarps,  the  utter  failure  to  find  any 
evidence  whatever  of  erosive  action  upon  the  surface  covered 
by  the  trap  caps,  becomes  full  warrant  for  affirming  that  the 
surface  has  never  been  exposed  to  such  agencies.  The  surface 
of  the  Animikio  slates  beneath  the  trap  presents  always  the 
characters  of  a  rock  freshly  ruptured  and  inveloped  immedi- 
ately in  the  trappean  magma.  Thus  again  the  sujiposition  that 
the  sheets  are  surface  flows  has  nothing  to  sustain  it,  and  all 
the  evidence  directly  supports  tho  view  that  the  sheets  are 
intrusive. 

I  THE   UPPER   CONTACT. 

In  most  of  the  intercalated  sheets  the  evidence  of  intrusion 
is  so  abundant  that  it  is  scarcely  necessary  to  go  into  the  de- 
tails of  their  upper  contact  with  the  enclosing  rock.  It  is  suf- 
ficient to  say  that  it  presents  no  essential  difference  from  the 
lower  contact.  The  trap  involves  the  sharp  angular  edges  of 
%e  slate,  where  they  have  been  ruptured  by  the  disturbance: 
apophyses  from  the  sheets  extend  (usually  for  not  more  than  a 
few  inches)  into  the  cracks  and  fissures  of  the  overlying  slates; 
and  angular  fragments  of  the  latter  ai'e  quite  frequently  found 
imbedded  in  the  trap.  As  has-been  stated,  the  opportunities 
of  examining  the  upper  contacts  of  the  trap  caps  are  few,  ow- 
ing to  the  general  removal  by  erosion  of  the  slates  which  once 
rested  upon  them.  In  the  vicinity  of  Port  Arthur,  however, 
some  remarkably  fine  sections  may  be  very  conveniently  ex- 
amined which  show  remnants  of  slates  still  reposing  upon  the 
trap  caps;  and  such  instances  would  be  doubtless  more  fre" 
quently  observed,  were  it  not  that  the  routes  of  travel  only  oc- 
casionally traverse  the  surface  of  the  caps,  and  that  the  latter 
are  very  commonly  covered  v,'ith  timber.  The  Port  Arthur  trap 
sheet  is  an  extensive  one,  and  not  only  underlies  the  town  but 
extends  inland  for  many  square  miles  and  skirts  the  shore  of 
Thunder  bay,  being  found  on  the  points  and  islands.  The  av- 
erage thickness  is  probably  about  50  feet.  On  the  surface  of 
this  sheet  at  several  places  may  be  seen  patches  of  slate,  ad- 
hering to  it,  and  frequently  sunk  down  into  it.  An  excellent 
section  is  afforded  in  the  railway  cutting  at  the  old  Canadian 
Pacific  railway  station  at  Water  street.  Port  Arthur.  This 
section  is  shown  in  Plate  VI.    (frontispiece).    The  upper  por- 


LACCOLITH  •    SILLS. 


43 


tionof  the  trap  sheet  is  very  dense  and  compact,  but  it  rapidly 
becomes  coarser  in  descendinj^r  through  the  thickness  of  the 
stieet.  The  plane  of  contact  is  on  the  whole,  even,  but  is  ra<;- 
«^ed  or  step  like  in  detail.  th(!  stejis  being  due  to  the  fractur 
ing  of  the  slates  by  the  invasion  of  the  trap.  The  nldfcs  oi'- 
altered  to  the  (liMance  of  orcr  a  foot  abort' the  trap.  In  other  sec 
tions  the  contact  is  still  more  step-like,  and  blocks  of  slate  may 
be  seen  to  he  sunk  down  within  the  trap.  The  surface  of  the 
sheet,  in  places  where  it  is  perfectly  evident  that  it  has  not 
been  affected  by  erosion,  is  peculiarly  domed  with  low  flat  un- 
OlVfftaOorfl.iikfSniieiHn- 


"r^ 


mm 


I't-afi  Can 


ipflTan,  PimiiHiEnjnrirain'T'iEG! 


Talus   chief  I  If  trap,  bleclta. 


RCHfrffif 
'/''•a/i  shfet 


T-alU't  chief  11/  shalti 
^^iss"  and  ■slaty  snnH stone 


~'  "-:— — \^¥^^;»>V 


^Ez.:-:rn=^A 


•^v" 

S 


/^  ake  lei'tl 


l''l;;6.     Diasfiiiiiniutic  sei'lloii  tlironu'li   McKay'-^   Moiiiitiiiii.   Fort    WiUiatn,  sliowiiiij 
the  relatiuii  or  Liicoolitii.'  Sills  to  the  Aiihiiikli"  sirivtu.    Pciile  I  inch.  ^'KU  fci'i. 

duladons.  The  entire  character  (Tf  the  up))er  surface  of  the 
Port  Arthur  sheet,  which  is  one  of  the  great  caps  of  the  region, 
although  locally  it  descends  to  the  level  of  the  lake,  is  strictly 
analogous  to  that  which  may  be  observed  on  the  projecting 
surface  (a  terrace  of  differential  degradation)  offered  by  the 
intecalated  sheet  at  the  Romar  Catholic  retreat  on  the  face  of 
McKay's  mountain  at  Fort  William.  This  sill  is  about  12  to  15 
feet  thick  (see  fig.  8)  and  has  the  same  locally  domed  orundul- 


44 


JJULI.KTIN    NO.    VIII. 


atory  surface  with  patches  of  th(;  overlying  slates  sunk  dowu 
into  it  and  partially  imbedded  in  it.  Tliis  domed  surface  of  the 
trap  sheets  has  been  also  noted  by  In^all*  in  othej*  localities. 

ALTKKATION  OF  THE  ENCLOHINC.  ROCKS. 

The  alteration  of  the  Animikie  slates  by  intrusive  masses  of 
the  same  petro*?raphical  character  and  fjeneral  dimensions,  is 
very  various  in  its  extent.  In  some  cases,  as  in  the  remarkable 
one  at  Pigeon  point,  described  by  Bayley.  it  amounts  to  a  com- 
plete fusion  of  the  invaded  rocks  and  the  mixture  of  such  fused 
rock  Avith  the  invading  magma,  giving  rise  to  peculiar  and  ex- 
ceedingly interesting  petrographical  types.  In  other  cases  the 
slates  have  only  been  altered  to  a  hornfels  to  the  extent  of  a 
few  inches,  or  at  most  a  few  feet  from  the  contact.  Sometimes 
it  requires  a  trained  petrographical  eye  to  detect  that  there 
has  been  any  alteration  whatever.  The  rocks  immediately  ad- 
jacent to  the  trap  sheets,  whether  at  their  lower  or  their  upper 
surface,  ai'e,  however,  always  altered  to  the  extent  at  least  of 
making  them  recognizable  as  hornfels.  They  are  hard  and 
dense  and  fre<]uently  resemble  somewhat  the  den.se  aphanitic 
facies  of  the  trap  at  the  contact,  so  that  some  geologists  have 
confessed  their  inability  to  distinguish  between  them.  This 
zone  has  not  been  subjected  to  systematic  petrographical  study, 
but  in  the  few  slides  which  the  writer  has  examined,  it  is  clear 
that  the  clastic  structure  of  the  rock  has  been  more  or  less  ob- 
scured by  re-crystallization,  and  that  the  rock  is  characterized 
by  the  abundant  development  of  minute  pleochroic  needles 
having  parallel  extinction,  and  resembling  green  hornblende 
but  for  the  latter  property.  This  alteration  is  sometimes 
associated  with  a  bleaching  of  the  rock;  and  in  some  cases  there 
are  suggestions  of  secondary  glass  having  been  formed.  But 
it  is  not  the  intention  of  the  writer  to  discuss  the  nature  of  the 
alteration.  It  is  sufficient  for  his  purpose  to  be  able  to  state 
that  there  is  a  prevalent  alteration  of  the  Animikie  slates,  both 
above  and  below  the  trap  sheets,  which  is  clearly  ascribable  to 
their  invasion  within  the  slates  as  ingneous  masses. 

SUMMARY. 

The  argument  may  be  summarized  briefly: 
I— The  trap  .sheets  associated  with  the  Animikie  strata  are 
not  volcanic  flows,  because  of  the  combination  of  the  following 

facts: 


'Op.  Cit. 


LACCOLITIC   SILLS. 


0 


1 


n. 
•1. 
'). 

0. 
7. 
8. 


They  aro   simpl*'  f,'eolo«;if'{il  units,  not   a  series   of  over 

lapping  sheets. 
They  arr^  Hat  with  uniform  thickness  over  areas  more  than 
one  hundred  square  miles  in  extent,  and  where  inclined, 
the  dip  is  due  essentially  to  faulting  and  tiltin/iif. 
There  are  no  pyroelastic  rocks  a.ssociated  with  them. 
They  are  nev(;r  glassy. 
They  are  never  aniygdaloidal. 
They  exhibit  no  tlow  structure. 
They  have  no  ropy  or  wrinkled  surface. 
They  have  no  lava  breccia  associated  with  them. 
They  caine  in  contact  with  the  shites  after  the  latter  were 
hard  and  brittle  and  had   acquired   their  cleavage;  yet 
they  never  repose  ujion  a  surface  which  has  been  exposed 
to  sub- aerial  weathering. 
11. — They  are  intrusive  sills  because  of  the  combination  of 
the  following  facts: 
1.     They  are  strictly  analogous  to  the   great  dykes  of   the 
region.     {")  In   their  general  relations  to  the  adjacent 
Tocks,   and  in  their  field  aspect,     (h)  In  that  both  the 
upper  and  lower  sides  of  the  sheets  have  the  facies  of 
a  dense  ay^hanitic  roclc.  which  grades  towards  the  middh' 
into  a  coarsely  crystalline  rock. 
They  have  a  practically  uniform  thickness  over  large  areas. 
The   columnar  structure  extends  from  lower  surface  to 
upper  surface,  as  it  does  from  wall  to  wall  in  the  dykes. 
They  intersected  the  strata  above  and  below  them  after 

the  latter  had  been  hard  and  brittle. 
They  may  be  observed  in  direct  continuity  with  dykes. 
They  pass  from  one  horizon  to  another. 
The  bottom  of  the  sedimentary  strata  above  them,  where- 

ever  it  is  observable,  is  a  freshly  ruptured  surface. 
Apophyses  of  the  trap  pass  from  the  main  shi^et  into  tlie 
cracks  of  the  slate  above  and  below. 

10.  The  trap  sheets,  particularly  at  the  upper  contact,  hold  in 

eluded  fragments  of  the  overlying  slates. 

11.  They  locally  alter  the  slates  above  and  below  them. 

GEOLOGICAL  CONSEQUENCES. 

In  the  lake  Superior  region  the  lowest  great  division  of  the 
Palaeozoic  is  known  to  the  Minnesota  survey  as  the  Taconic 
system.  This  system  embraces  two  groups,  viz:  The  Animikie 
and  the  Keweenian  ^Keweenawan).  Between  the  Animikie  and 
the  Keweenian  there  is  an  interval  of  erosion  and  a  consequent 


2. 
3. 


5. 
6. 
8. 

9. 


4f') 


BULLETIN   NO.    V\l\. 


unconformity.  In  the  opinion  of  tlie  writer  this  uncont'orniity 
represonts  a  mucli  moro  important  interval  than  has  been  com- 
monly supposed.  Tlio  slight  amount  of  distiirbance,  which 
alTected  tlio  Animiki<»  roclcs  prior  to  the  deposition  of  the  Ke- 
woonian. leaves  the  two  sets  of  rocks  in  nearly  parallel  bedding; 
and  the  fact  of  uplift  and  long  continued  erosion  of  the;  Animikie, 
prior  to  the  dei)osition  of  the  Keweenian,  is  only  realized  by  a 
careful  study  of  the  contacts  of  the  latter  with  the  lower  rocks. 
The  belief  that  the  two  groups  of  rocks  were  in  stratigraphic 
continuity,  or  only  separated  by  a  slight  break,  seems  to  have 
led  the  earlier  geologists  to  regard  the  trap  sheets  associated 
with  the  Animikie  as  simply  a  manifestation  of  the  volcanic 
activity  which  prevailed  in  Keweenian  times. 

The  propositions  established  in  this  paper  tend  to  empha- 
size the  contrast  between  the  two  divisions  of  the  Taconic 
system.  Not  only  is  there  a  pronounced  stratigraphic  break 
between  the  Animikie  and  the  Keweenian,  but  the  earlier  group 
was  deposited  in  a  time  of  quiescence  entirely  free  from  vol- 
canic disturbances,  while  during  the  deposition  of  the  latter, 
volcanic  extravasations  were  poured  out  in  no  sparing  .meas- 
ure. 

The  contrast  which  is  thus  presented  by  the  two  divisions  of 
the  Taconic  system  is  analogous  to  that  which  obtains  between 
the  two  divisions  of  the  Ontarian  system  in  the  Archa3an.  The 
lower  group  of  the  Ontarian  system,  the  Coutchiching.  is  a 
great  series  of  metamorphic  sediments  free  from  volcanic  rocks;* 
this  is  followed  by  the  Keewatin  group,  in  which  volcanic  rocks 
and  their  metamorphic  products  predominate. 

The  recognition  of  the  trap  sheets  as  laccolitic  sills  opens  up 
to  students  of  American  geology  a  large  and  interesting  prob- 
lem of  a  phase  of  crust-building  which  is  here  barely  touched 
upon.  The  lateral  injection  of  great  sheets  of  molten  matter 
within  the  bedding  planes  of  horizontal  series  of  rocks  is  of 
course  a  well  known  fact.  But  it  is  also  a  problem  in  physical 
geology  which  has  not  been  thoroughly  worked  out;  and  the 
prevalence  and  importance  of  the  process  of  lateral  intrusion, 
as  a  factor  in  the  development  of  the  earth's  crust,  is  far  from 
being  sufficiently  recognized  in  geological  literature,  notwith- 
standing the  impetus  which  has  been  given  to  this  line  of  in- 
quiry by  Gilbert's  classic  work  on  the  Henry  mountains.! 

*Coniparo.  howevpr.  N.  II.  and  11.  V.  Wlnchell  (Iron  Ores  of  Minnesota,  pp  11-24)  who 
show  ttiiit  probably  the  verniilloii  (OoutchichinK) sediments  are  modifleu  ounditlons 
of  older  sediments  of  like  origin  and  cbaracter  with  those  of  the  Keewatio. 

tOonipare  also  N.  H.  Windiell.  Some  thoushts  on  eruptli^e  rocks  with  special  refer- 
once  to  those  of  Minnesota,  A.  A.  A.  S.,  1888,  pp.  212-281. 


laccomtk;  sills. 


47 


,.* 


To  investigators  dpsirons  of  attiiolcinf^  this  important  prob- 
iotu  no  more  successful  liold.  and  no  inoro  iii)untiant  opportun- 
ities for  unobscurod  obs(!rvation  of  fr(»sli  rocl^s.  could  be  de- 
sired than  those  presented  by  the  northwest  coast  of  lake  Su- 
l)erior. 

The  laccolitic  charact<'r  of  tlic  trap  ca])s  of  the  region  of 
course  implies  an  extensive  erosion  anil  removal  of  tlieoncc  ovi'r- 
lyinff  strata.  How  thick  this  volume  of  rock  origiiuiUy  was 
we  have  little  means  of  knowing.  It  seems  not  improljable, 
however,  that  over  much  of  the  Animikie  region  now  capped 
with  tliese  thick  sills,  not  only  was  there  a  very  consideral)le 
thickness  of  the  u])p(M'  portions  of  the  Animikie.  l)iit  also  that 
the  Keweenian  strata  f)ccui)ied  the  same  area. 

The  sills  are  not  only  of  later  age;  than  the  Animikie.  but  they 
appear  to  be  of  distinctly  post- Keweenian  age.  As  has  been 
shown,  the  sills  jiass  from  the  hori^con  of  the  Animikie  to  the 
Keweenian.  and  the  same  features  which  prov(>  them  to  be  in- 
trusive in  the  Animikie  show  then  also  to  be  intrusive  in  the 
Keweenian.  This  is  true,  not  only  of  the  Keweenian  of  the 
Canadian  side  of  lake  Superior,  but  also  of  the  Keweenian  of 
the  Minnesota  coast.  While  fully  recognizing  the  essentially 
volcanic  character  of  the  Keweenian  series  the  writer  has  no 
hesitation  in  stating  his  0/H///0/*  that  many  t)f  the  heavy  sheets 
of  dark  diabase  or  gabbro,  whi(;h  prevail  on  the  Minnesota 
coast,  particularly  in  its  eastern  i)ortion.  and  which  have  been 
described  and  referred  to  by  former  observers  as  volcanic  flows 
of  Keweenian  age.  are  laccolitic  sills.  The.se  have  been  inject- 
ed along  the  bedding  planes  of  the  volcanic  sheets  of  the  Ke- 
weenian, just  as  they  have  been  between  the  sedimentary  planes 
of  the  Animikie.  These  are,  therefore,  of  post- Keweenian  age. 
Many  of  these  sheets  are  petrographically  identical  with  those 
of  the  Animikie  province,  while  others  present  differences  in 
mineralogical  detail  and  general  aspect. 

The  recognition  of  the  laccolitic  character  of  the  trap  sheets 
associated  with  the  Animikie  rocks,  tlnd  of  their  analogues 
associated  with  the  Keweenian,  involves  the  recognition  also 
of  a  distinct  and  important  event  in  the  geological  history  of 
the  lake  Superior  region.  The  intrusion  of  these  sills  is  an 
event  subsequent  to  the  formation  of  the  Animikie  and  Keween- 
ian rocks.  How  much  later  in  age  they  are  we  do  not  yet 
know,  as  there  are  no  other  palaeozoic  rocks  in  the  region  to 
serve  as  a  proximate  limit.  They  may.  indeed,  be  much  later 
than  the  Taconic,  and  may  possibly  correspond  in  age  with  the 


48 


mil.LETlN   NO.    VIII. 


jfreat  series  of  Iriij)  rocUs  ititnuli'd  in  llu;  Siliiriun  njcks  of 
Quebec,  of  which  mount  Koyal,  at  Montreal,  is  a  well  known 
in^>tance.  In  view  of  the  uncertainty  as  to  thoir  aj<o,  and 
of  their  individuality  as  an  i«;noous  formatitin  distinct  from  the 
Anin)iki<!  and  tlu*  Keweenian  strata,  these  trap  shetds  secun  to 
re»|uir(!  a  distin(!tivo  name.  To  continue  to  refer  to  them  as 
the  Animikie  traps,  is  to  cultivate  a  confusion  of  ideas,  and  to 
refer  to  them  as  the  "trap  sheets  asso(riated  with  the  Animikie" 
is  too  cumbersome.  Thon^fore.  in  the  interest  of  clear  ideas, 
and  in  order  to  si<?nalize  the  discovery  of  an  important  event 
in  tlu!  <.reolo«;ical  history  of  the  rej^ion,  the  writc^r  propDses  to 
designate  these  ti"ap  sheets  as  the  Logan  sills,  in  honor  of 
the  late  Sir  William  E.  Logan,  to  whose  tireless  research 
American  geology  owes  an  enduring  debt  of  gratitude,  and 
whose  ex])lorations  gave  us  our  first  knowledge  of  the  physical 
geology  of  lake  Superior,  and  of  the  rocks  which  form  the  sub- 
ject of  this  paper. 


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